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Hemoglobin, hematocrit and WBC are just the beginning–don’t overlook erythrocytes, leukocytes and thrombocytes for important assessment data.

If you don’t use it you lose it! That aptly applies to interpreting the complete blood count (CBC) and differential (diff). Most of us are well acquainted with hemoglobin, hematocrit and white blood cells (WBC), but perhaps the rest of those numbers are insignificant to the particular patient being tested … or are they? What is the meaning of those other components of the CBC and diff?

Blood Components

Blood is made of two major components-plasma and cells. Plasma is the liquid part of the blood in which the formed cells are suspended. The plasma consists of water, plasma proteins (a few of which are serum albumin and globulin and fibrinogen), and other constituents. Plasma makes up more than half of the total blood volume.

The cells are the blood components that will be discussed in this review. Cells of the blood include the erythrocytes, which are the red blood cells (RBC); the leukocytes, which are the WBC; and the thrombocytes, also known as platelets.

Blood cells are produced in the bone marrow by a process called hematopoiesis. Red blood cell production is regulated by erythropoietin, a hormone released by the kidneys. When blood oxygen is low, erythropoietin stimulates the bone marrow to produce more RBCs.

What Does the CBC Test Analyze?

The CBC tests for the amount of RBCs, hemoglobin, hematocrit, reticulocytes, mean corpuscular volume, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration. Usually, platelets will also be checked with the CBC.

Red blood cells: RBCs are the number of erythrocytes in 1 cubic mm of whole blood. The RBC count will be low with iron deficiency, blood loss, hemolysis and bone marrow suppression. Increases may be found when one moves to a higher altitude or after prolonged physical exercise, and can also reflect the body’s attempt to compensate for hypoxia. Normal levels in men and women are 4.6 million-5.9 million and 4.1 million-5.4 million, respectively.

Polycythemia vera, a pathologic condition which is a proliferative disease of the bone marrow, causes an increase in total RBCs as well as an elevation in white cells and platelet count. Mild polycythemia may be corrected by increasing vascular fluid volume, while more severe cases require frequent phlebotomies or even radiation or chemotherapy to suppress bone marrow production.

Mature RBCs have a lifespan of about 120 days. In hemolytic anemia, the cell life span may be shorter. It is important to know this for patients desiring autologous transfusion (receiving one’s own blood), as red cell survival may be an issue. A hemolytic anemia patient should seek further medical advice before making an autologous donation.

Autologous transfusions are often considered before surgery to reduce the risk of blood-borne infections and transfusion reactions. Patients should deposit their blood up to 6 weeks prior to surgery.

Hemoglobin: Hemoglobin is the oxygen-carrying pigment of red cells. There are millions of hemoglobin molecules in each red cell. This blood component carries oxygen from the lungs to the body tissues. Decreases in hemoglobin occur for the same reasons as decreased RBCs. Normal levels in men and women are 14-18 g/dl and 12-16 g/dl respectively.

Hematocrit: The test for hematocrit measures the volume of cells as a percentage of the total volume of cells and plasma in whole blood. This percentage is usually three times greater than the hemoglobin. After hemorrhage or excessive intravenous fluid infusion, the hematocrit will be low. If the patient is dehydrated, the hematocrit will be increased. Normal levels in men and women are 42 percent-52 percent and 37 percent-47 percent respectively.

Reticulocyte: These are the new cells released by the bone marrow. The reticulocyte count is therefore used to assess bone marrow function and can indicate the rate and production of RBCs. Normal to slightly elevated reticulocyte counts may occur with anemia demonstrating an underproduction of red cells (such as with iron or folate deficiencies), depending on the staging of the disease. Elevated levels may indicate blood loss or hemolysis. Normal levels are 0.5 percent to 1.5 percent.


Indices measure the average characteristics of the erythrocyte. The indices usually noted include the mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), the mean corpuscular hemoglobin concentration (MCHC) and red cell distribution width (RDW).

MCV: This measures the average size of the RBC and can be calculated by dividing hematocrit X10 by RBC count. Normal values are 80-100 fL.

Low values indicate the cells are microcytic (small cells) and are often evident with conditions such as iron deficiency, lead poisoning and the thalassemias. High values greater than 100 fL indicate macrocytic cells (large cells), and are found with such conditions as megaloblastic anemia, folate or Vitamin B12 deficiency, liver disease, post-splenectomy, chemotherapy or hypothyroidism. The MCV can be normal with a low hemoglobin if the patient is hypovolemic or has had an acute blood loss.

MCH: MCH is the average weight of hemoglobin per red cell. Normal level is 27 to 311 picograms (pg) or 28-33 pg, depending on the reference.2

MCHC: MCHC is the average concentration of hemoglobin per erythrocyte. Normal levels can be seen with acute blood loss, folate and Vitamin B12 deficiency; these cells will still be normochromic. Hypochromic or “pale cells” will be seen with conditions such as iron deficiency and the thalassemias. Normal levels are 32 percent-36 percent.1,2

RDW: This index is a quantitative estimate of the uniformity of individual cell size. Elevated levels may indicate iron deficiency or other conditions with a wide distribution of various cell sizes. Normal levels are 11.5 percent to 14.5 percent.1


Platelets, also known as thrombocytes, are small elements formed in the red bone marrow. They are actually fragments of megakaryocyte cytoplasm (precursor cell to the platelet.) Platelets help to control bleeding. There are two means by which platelets are able to do this: one is by forming an occlusion at small injurious openings in blood vessels; and the second by a thromboplastic function which stimulates the coagulation cascade. Both platelet number (measurable by platelet count) and platelet function (not measurable by platelet count) play a role in the effectiveness of the platelet in controlling bleeding. Note that platelet count measures only platelet number, not function.

In the cases of thrombocytopenia, the patient will have decreased platelets and can experience severe bleeding. Thrombocytopenia may occur for many reasons, a few of which are:

  • aplastic anemia, in which the patient experiences loss of bone marrow function;
  • drug-induced; or
  • leukemia, in which the bone marrow is replaced by malignant cells.

Certain conditions also reduce platelet function.

Many conditions can elevate platelet number, a few of which include:

  • essential thrombocythemia,
  • chronic leukemia (depending on stage and therapy),
  • post-splenectomy,
  • iron deficiency anemia,
  • malignancy, and
  • chronic infection or inflammation. The reader should remember that the staging of the disease process and the therapeutic regimen can cause platelet number to fluctuate.

Other conditions may enhance platelet function, a few of which are atherosclerosis, diabetes, smoking and elevated lipid and cholesterol levels. These situations can enhance the patient’s chances of developing thrombosis. The normal level of platelets is 150,000-350,000/cubic mm.

White Blood Cells

WBCs, also known as leukocytes, are larger in size and less numerous than red cells. They develop from stem cells in the bone marrow. WBC function involves the response to an inflammatory process or injury. Normal levels of WBCs for men and women are 4,300-10,800/cubic mm.

When the white count is abnormal, the differential segment can measure the percentage of the various types of white cells present. Differential counts add up to 100 percent. The differential usually includes neutrophils, bands, eosinophils, monocytes and lymphocytes.

Though the discussion below lists each differential cell and describes increases or decreases in percentage in response to various stimuli, the reader must also remember that most of these percentages can also fluctuate in patients with certain kinds of leukemia and other pathologic conditions.Neutrophils: The function of neutrophils is to destroy and ingest bacteria. Neutrophils arrive first at the site of inflammation; therefore their numbers will increase greatly immediately after an injury or during the inflammatory process. Their life span is approximately 10 hours, then a cycle of replenishing neutrophils must occur. Besides during inflammation, neutrophils increase with such conditions as stress, necrosis from burns and heart attack. Normal levels range from 45 percent-74 percent.

Bands: These are occasionally referred to as “stabs” and are immature neutrophils which are released after injury or inflammation. The presence of bands indicates that an inflammatory process is occurring. An increase in the release of immature cells is known as a “shift to the left.” In the days of written reports, lab personnel would write the bands in the left margin, hence the lasting name some sources claim, which represents an increase of bands or stabs.2 However, other references say the shift to the left refers to the early release of younger white cells such as bands and metamyelocytes from the bone marrow reserve into the blood stream (a shift from the right, meaning mature cells, toward the left of the maturation series, meaning less mature cells). Normal level ranges from 0 percent-4 percent.

Eosinophils: These are found in such areas as skin and the airway in addition to the bloodstream. They increase in number during allergic and inflammatory reactions and parasite infections. Normal blood levels range from 0 percent-7 percent.

Basophils: Called basophils when found in the blood, these cells are also known as “mast” cells when found in the tissues. Tissue basophils are found in the gastrointestinal and respiratory tracts and the skin. They contain heparin and histamine and are believed to be involved in allergic and stress situations. Basophils may contribute to preventing clotting in microcirculation. Normal blood levels range from 0 percent-2 percent.

Monocytes: These cells arrive at the site of injury in about five hours or more.3 The monocytes are phagocytic cells that remove foreign materials such as injured and dead cells, microorganisms and other particles from the site of injury, particularly during viral or bacterial infections. Normal levels, which vary depending on the source, range from 2 percent-8 percent3 to 4 percent-10 percent.2

Lymphocytes: Lymphocytes fight viral infections; B cells and T cells are two major types. Lymphocytes have a key role in the formation of immunoglobins (humoral immunity) and also provide cellular immunity. Normal levels range from 16 percent-45 percent.


  1. Kee, J. (1999). Laboratory and diagnostic tests with nursing implications. (5th ed.). Stamford, CT: Appleton & Lange.
  2. Corbett, J. (1996). Laboratory tests & diagnostic procedures with nursing diagnosis. (4th ed.). Stamford, CT: Appleton & Lange, (p. 59).
  3. Porth, C. (1994). Pathophysiology: Concepts of altered health states. (4th ed.). Philadelphia: JB Lippincott Co.
Sours: https://www.elitecme.com/resource-center/laboratory/interpreting-the-complete-blood-count-and-differential

Test Overview

A complete blood count (CBC) gives important information about the kinds and numbers of cells in the blood, especially red blood cells, white blood cells, and platelets. A CBC helps your doctor check any symptoms, such as weakness, fatigue, or bruising, you may have. A CBC also helps him or her diagnose conditions, such as anemia, infection, and many other disorders.

A CBC test usually includes:

  • White blood cell (WBC, leukocyte) count. White blood cells protect the body against infection. If an infection develops, white blood cells attack and destroy the bacteria, virus, or other organism causing it. White blood cells are bigger than red blood cells but fewer in number. When a person has a bacterial infection, the number of white cells rises very quickly. The number of white blood cells is sometimes used to find an infection or to see how the body is dealing with cancer treatment.
  • White blood cell types (WBC differential). The major types of white blood cells are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Immature neutrophils, called band neutrophils, are also part of this test. Each type of cell plays a different role in protecting the body. The numbers of each one of these types of white blood cells give important information about the immune system. Too many or too few of the different types of white blood cells can help find an infection, an allergic or toxic reaction to medicines or chemicals, and many conditions, such as leukemia.
  • Red blood cell (RBC) count. Red blood cells carry oxygen from the lungs to the rest of the body. They also carry carbon dioxide back to the lungs so it can be exhaled. If the RBC count is low (anemia), the body may not be getting the oxygen it needs. If the count is too high (a condition called polycythemia), there is a chance that the red blood cells will clump together and block tiny blood vessels (capillaries). This also makes it hard for your red blood cells to carry oxygen.
  • Hematocrit (HCT, packed cell volume, PCV). This test measures the amount of space (volume) red blood cells take up in the blood. The value is given as a percentage of red blood cells in a volume of blood. For example, a hematocrit of 38 means that 38% of the blood's volume is made of red blood cells. Hematocrit and hemoglobin values are the two major tests that show if anemia or polycythemia is present.
  • Hemoglobin (Hgb). The hemoglobin molecule fills up the red blood cells. It carries oxygen and gives the blood cell its red colour. The hemoglobin test measures the amount of hemoglobin in blood and is a good measure of the blood's ability to carry oxygen throughout the body.
  • Red blood cell indices. There are three red blood cell indices: mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). They are measured by a machine and their values come from other measurements in a CBC. The MCV shows the size of the red blood cells. The MCH value is the amount of hemoglobin in an average red blood cell. The MCHC measures the concentration of hemoglobin in an average red blood cell. These numbers help in the diagnosis of different types of anemia. Red cell distribution width (RDW) can also be measured which shows if the cells are all the same or different sizes or shapes.
  • Platelet (thrombocyte) count. Platelets (thrombocytes) are the smallest type of blood cell. They are important in blood clotting. When bleeding occurs, the platelets swell, clump together, and form a sticky plug that helps stop the bleeding. If there are too few platelets, uncontrolled bleeding may be a problem. If there are too many platelets, there is a chance of a blood clot forming in a blood vessel. Also, platelets may be involved in hardening of the arteries (atherosclerosis).
  • Mean platelet volume (MPV). Mean platelet volume measures the average amount (volume) of platelets. Mean platelet volume is used along with platelet count to diagnose some diseases. If the platelet count is normal, the mean platelet volume can still be too high or too low.

Your doctor may order a blood smear test to be done at the same time as a CBC but it is not part of the regular CBC test. In this test, a drop of blood is spread (smeared) on a slide and stained with a special dye. The slide is looked at under a microscope. The number, size, and shape of red blood cells, white blood cells, and platelets are recorded. Blood cells with different shapes or sizes can help diagnose many blood diseases, such as leukemia, malaria, or sickle cell disease.

Why It Is Done

A complete blood count may be done to:

  • Find the cause of symptoms such as fatigue, weakness, fever, bruising, or weight loss.
  • Check for anemia.
  • See how much blood has been lost if there is bleeding.
  • Diagnose polycythemia.
  • Check for an infection.
  • Diagnose diseases of the blood, such as leukemia.
  • Check how the body is dealing with some types of drug or radiation treatment.
  • Check how abnormal bleeding is affecting the blood cells and counts.
  • Screen for high and low values before a surgery.
  • See if there are too many or too few of certain types of cells. This may help find other conditions, such as too many eosinophils may mean an allergy or asthma is present.

A complete blood count may be done as part of a regular physical examination. A blood count can give valuable information about the general state of your health.

How To Prepare

You do not need to do anything before having this test.

How It Is Done

Your health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure to the site and then a bandage.

If this blood test is done on a baby, a heel stick will be done instead of a blood draw from a vein.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.


There is very little chance of a problem from having a blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.


A complete blood count (CBC) gives important information about the kinds and numbers of cells in the blood, especially red blood cells, white blood cells, and platelets. A CBC helps your doctor check any symptoms, such as weakness, fatigue, or bruising, you may have. A CBC also helps him or her diagnose conditions, such as anemia, infection, and many other disorders.


The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Normal values for the complete blood count (CBC) tests depend on age, sex, how high above sea level you live, and the type of blood sample. Your doctor may use all the CBC values to check for a condition. For example, the red blood cell (RBC) count, hemoglobin (Hgb), and hematocrit (HCT) are the most important values needed to tell whether a person has anemia, but the red blood cell indices and the blood smear also help with the diagnosis and may show a possible cause for the anemia.

To see if the white blood cell (WBC, leukocyte) count is good and how the cells look on the smear, your doctor will look at both the number (WBC count) and the WBC differential. To see whether there are too many or too few of a certain type of cell, your doctor will look at the total count and the percentage of that particular cell. There are normal values for the total number of each type of white cell.

Pregnancy can change these blood values. Your doctor will talk with you about normal values during each trimester of your pregnancy.

Men and non-pregnant women:

5,000–10,000 WBCs per cubic millimetre (mm 3) or 5.0–10.0 x 10 9 WBCs per litre (L)



Band neutrophils:











4.5–5.5 million RBCs per microliter (mcL) or 4.5–5.5 x 10 12/liter (L)


4.0–5.0 million RBCs per mcL or 4.0–5.0 x 10 12/L


3.8–6.0 million RBCs per mcL or 3.8–6.0 x 10 12/L


4.1–6.1 million RBCs per mcL or 4.1–6.1 x 10 12/L


42%–52% or 0.42–0.52 volume fraction


36%–48% or 0.36–0.48 volume fraction


29%–59% or 0.29–0.59 volume fraction


44%–64% or 0.44–0.64 volume fraction


14–17.4 grams per deciliter (g/dL) or 140–174 grams per liter (g/L)


12–16 g/dL or 120–160 g/L


9.5–20.5 g/dL or 95–205 g/L


14.5–24.5 g/dL or 145–245 g/L

In general, a normal hemoglobin level is about one-third the value of the hematocrit.

Mean corpuscular volume (MCV)—Adults:

84–96 femtoliters (fL)

Mean corpuscular hemoglobin (MCH)—Adults:

28–34 picograms (pg) per cell

Mean corpuscular hemoglobin concentration (MCHC)—Adults:

32–36 grams per deciliter (g/dL)




140,000–400,000 platelets per mm 3 or 140–400 x 10 9/L


150,000–450,000 platelets per mm 3 or 150–450 x 10 9/L


7.4–10.4 mcm 3 or 7.4–10.4 fL


7.4–10.4 mcm 3 or 7.4–10.4 fL


Blood cells are normal in shape, size, colour, and number.

High values

Red blood cell (RBC)

  • Conditions that cause high RBC values include smoking, exposure to carbon monoxide, long-term lung disease, kidney disease, some cancers, certain forms of heart disease, alcohol use disorder, liver disease, a rare disorder of the bone marrow (polycythemia vera), or a rare disorder of hemoglobin that binds oxygen tightly.
  • Conditions that affect the body's water content can also cause high RBC values. These conditions include dehydration, diarrhea or vomiting, excessive sweating, and the use of diuretics. The lack of fluid in the body makes the RBC volume look high; this is sometimes called spurious polycythemia.

White blood cell (WBC, leukocyte)

  • Conditions that cause high WBC values include infection, inflammation, damage to body tissues (such as a heart attack), severe physical or emotional stress (such as a fever, injury, or surgery), kidney failure, lupus, tuberculosis (TB), rheumatoid arthritis, malnutrition, leukemia, and diseases such as cancer.
  • The use of corticosteroids, underactive adrenal glands, thyroid gland problems, certain medicines, or removal of the spleen can also cause high WBC values.


  • High platelet values may be seen with bleeding, iron deficiency, some diseases like cancer, or problems with the bone marrow.

Low values

Red blood cell (RBC)

  • Anemia lowers RBC values. Anemia can be caused by heavy menstrual bleeding, stomach ulcers, colon cancer, inflammatory bowel disease, some tumours, Addison's disease, thalassemia, lead poisoning, sickle cell disease, or reactions to some chemicals and medicines. A low RBC value may also be seen if the spleen has been taken out.
  • A lack of folic acid or vitamin B12 can also cause anemia, such as pernicious anemia, which is a problem with absorbing vitamin B12.
  • The RBC indices value and a blood smear may help find the cause of anemia.

White blood cell (WBC, leukocyte)


  • Low platelet values can occur in pregnancy or immune thrombocytopenic purpura (ITP) and other conditions that affect how platelets are made or that destroy platelets.
  • A large spleen can lower the platelet count.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • If the elastic band was on your arm a long time while the blood sample was taken.
  • Taking medicines that can cause low platelet levels. Some examples of the many medicines that cause low platelet levels include steroids, some antibiotics, thiazide diuretics, chemotherapy medicines, quinidine, and meprobamate.
  • A very high white blood cell count or high levels of a type of fat (triglycerides). These can cause falsely high hemoglobin values.
  • Having an enlarged spleen, which may cause a low platelet count (thrombocytopenia) or a low white blood cell count. An enlarged spleen may be caused by certain types of cancer.
  • Pregnancy, which normally causes a low RBC value and less often a high WBC value.



  1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.


Current as of: December 9, 2019

Author: Healthwise Staff
Medical Review:
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine

Sours: https://www.healthlinkbc.ca/medical-tests/hw4260
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Complete blood count

Routine laboratory test of blood cells

A complete blood count (CBC), also known as a full blood count (FBC), is a set of medical laboratory tests that provide information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cells and platelets, the concentration of hemoglobin, and the hematocrit (the volume percentage of red blood cells). The red blood cell indices, which indicate the average size and hemoglobin content of red blood cells, are also reported, and a white blood cell differential, which counts the different types of white blood cells, may be included.

The CBC is often carried out as part of a medical assessment, and can be used to monitor health or diagnose diseases. The results are interpreted by comparing them to reference ranges, which vary with sex and age. Conditions like anemia and thrombocytopenia are defined by abnormal complete blood count results. The red blood cell indices can provide information about the cause of a person's anemia such as iron deficiency and vitamin B12 deficiency, and the results of the white blood cell differential can help to diagnose viral, bacterial and parasitic infections and blood disorders like leukemia. Not all results falling outside of the reference range require medical intervention.

The CBC is performed using basic laboratory equipment or an automated hematology analyzer, which counts cells and collects information on their size and structure. The concentration of hemoglobin is measured, and the red blood cell indices are calculated from measurements of red blood cells and hemoglobin. Manual tests can be used to independently confirm abnormal results. Approximately 10–25% of samples require a manual blood smear review,[5] in which the blood is stained and viewed under a microscope to verify that the analyzer results are consistent with the appearance of the cells and to look for abnormalities. The hematocrit can be determined manually by centrifuging the sample and measuring the proportion of red blood cells, and in laboratories without access to automated instruments, blood cells are counted under the microscope using a hemocytometer.

In 1852, Karl Vierordt published the first procedure for performing a blood count, which involved spreading a known volume of blood on a microscope slide and counting every cell. The invention of the hemocytometer in 1874 by Louis-Charles Malassez simplified the microscopic analysis of blood cells, and in the late 19th century, Paul Ehrlich and Dmitri Leonidovich Romanowsky developed techniques for staining white and red blood cells that are still used to examine blood smears. Automated methods for measuring hemoglobin were developed in the 1920s, and Maxwell Wintrobe introduced the Wintrobe hematocrit method in 1929, which in turn allowed him to define the red blood cell indices. A landmark in the automation of blood cell counts was the Coulter principle, which was patented by Wallace H. Coulter in 1953. The Coulter principle uses electrical impedance measurements to count blood cells and determine their sizes; it is a technology that remains in use in many automated analyzers. Further research in the 1970s involved the use of optical measurements to count and identify cells, which enabled the automation of the white blood cell differential.


Blood is composed of a fluid portion, called plasma, and a cellular portion that contains red blood cells, white blood cells and platelets.[note 1][7] The complete blood count evaluates the three cellular components of blood. Some medical conditions, such as anemia or thrombocytopenia, are defined by marked increases or decreases in blood cell counts.[8] Changes in many organ systems may affect the blood, so CBC results are useful for investigating a wide range of conditions. Because of the amount of information it provides, the complete blood count is one of the most commonly performed medical laboratory tests.[9][10][11]

The CBC is often used to screen for diseases as part of a medical assessment.[12] It is also called for when a healthcare provider suspects a person has a disease that affects blood cells, such as an infection, a bleeding disorder, or some cancers. People who have been diagnosed with disorders that may cause abnormal CBC results or who are receiving treatments that can affect blood cell counts may have a regular CBC performed to monitor their health,[4][12] and the test is often performed each day on people who are hospitalized.[13] The results may indicate a need for a blood or platelet transfusion.[14]

The complete blood count has specific applications in many medical specialties. It is often performed before a person undergoes surgery to detect anemia, ensure that platelet levels are sufficient, and screen for infection,[15][16] as well as after surgery, so that blood loss can be monitored.[12][17] In emergency medicine, the CBC is used to investigate numerous symptoms, such as fever, abdominal pain, and shortness of breath,[18][19][20] and to assess bleeding and trauma.[21][22] Blood counts are closely monitored in people undergoing chemotherapy or radiation therapy for cancer, because these treatments suppress the production of blood cells in the bone marrow and can produce severely low levels of white blood cells, platelets and hemoglobin.[23] Regular CBCs are necessary for people taking some psychiatric drugs, such as clozapine and carbamazepine, which in rare cases can cause a life-threatening drop in the number of white blood cells (agranulocytosis).[24][25] Because anemia during pregnancy can result in poorer outcomes for the mother and her baby, the complete blood count is a routine part of prenatal care;[26] and in newborn babies, a CBC may be needed to investigate jaundice or to count the number of immature cells in the white blood cell differential, which can be an indicator of sepsis.[27][28]

The complete blood count is an essential tool of hematology, which is the study of the cause, prognosis, treatment, and prevention of diseases related to blood.[29] The results of the CBC and smear examination reflect the functioning of the hematopoietic system—the organs and tissues involved in the production and development of blood cells, particularly the bone marrow.[9][30] For example, a low count of all three cell types (pancytopenia) can indicate that blood cell production is being affected by a marrow disorder, and a bone marrow examination can further investigate the cause.[31] Abnormal cells on the blood smear might indicate acute leukemia or lymphoma,[30] while an abnormally high count of neutrophils or lymphocytes, in combination with indicative symptoms and blood smear findings, may raise suspicion of a myeloproliferative disorder or lymphoproliferative disorder. Examination of the CBC results and blood smear can help to distinguish between causes of anemia, such as nutritional deficiencies, bone marrow disorders, acquired hemolytic anemias and inherited conditions like sickle cell anemia and thalassemia.[32][33]

The reference ranges for the complete blood count represent the range of results found in 95% of apparently healthy people.[note 2][35] By definition, 5% of results will always fall outside this range, so some abnormal results may reflect natural variation rather than signifying a medical issue.[36] This is particularly likely if such results are only slightly outside the reference range, if they are consistent with previous results, or if there are no other related abnormalities shown by the CBC.[37] When the test is performed on a relatively healthy population, the number of clinically insignificant abnormalities may exceed the number of results that represent disease.[38] For this reason, professional organizations in the United States, United Kingdom and Canada recommend against pre-operative CBC testing for low-risk surgeries in individuals without relevant medical conditions.[15][39][40] Repeated blood draws for hematology testing in hospitalized patients can contribute to hospital-acquired anemia and may result in unnecessary transfusions.[38]


CBC performed by the fingerstickmethod, using an AbbottCell-Dyn 1700 automated analyzer

The sample is collected by drawing blood into a tube containing an anticoagulant—typically EDTA—to stop its natural clotting.[41] The blood is usually taken from a vein, but when this is difficult it may be collected from capillaries by a fingerstick, or by a heelprick in babies.[42][43] Testing is typically performed on an automated analyzer, but manual techniques such as a blood smear examination or manual hematocrit test can be used to investigate abnormal results.[44] Cell counts and hemoglobin measurements are performed manually in laboratories lacking access to automated instruments.[45]


On board the analyzer, the sample is agitated to evenly distribute the cells, then diluted and partitioned into at least two channels, one of which is used to count red blood cells and platelets, the other to count white blood cells and determine the hemoglobin concentration. Some instruments measure hemoglobin in a separate channel, and additional channels may be used for differential white blood cell counts, reticulocyte counts and specialized measurements of platelets.[46][47][48] The cells are suspended in a fluid stream and their properties are measured as they flow past sensors in a technique known as flow cytometry.[note 3][49][52]Hydrodynamic focusing may be used to isolate individual cells so that more accurate results can be obtained: the diluted sample is injected into a stream of low-pressure fluid, which causes the cells in the sample to line up in single file through laminar flow.[53][54]

Schematic of the Coulter principle. A particle suspended in a conductive medium passes through an aperture, causing an increase in impedance
The Coulter principle—the transient current drop is proportional to the particle volume

To measure the hemoglobin concentration, a reagent chemical is added to the sample to destroy (lyse) the red cells in a channel separate from that used for red blood cell counts. On analyzers that perform white blood cell counts in the same channel as hemoglobin measurement, this permits white blood cells to be counted more easily.[55] Hematology analyzers measure hemoglobin using spectrophotometry and are based on the linear relationship between the absorbance of light and the amount of hemoglobin present. Chemicals are used to convert different forms of hemoglobin, such as oxyhemoglobin and carboxyhemoglobin, to one stable form, usually cyanmethemoglobin, and to create a permanent colour change. The absorbance of the resulting colour, when measured at a specific wavelength—usually 540 nanometres—corresponds with the concentration of hemoglobin.[56][57]

Sensors count and identify the cells in the sample using two main principles: electrical impedance and light scattering.[58] Impedance-based cell counting operates on the Coulter principle: cells are suspended in a fluid carrying an electric current, and as they pass through a small opening (an aperture), they cause decreases in current because of their poor electrical conductivity. The amplitude of the voltage pulse generated as a cell crosses the aperture correlates with the amount of fluid displaced by the cell, and thus the cell's volume,[59][60] while the total number of pulses correlates with the number of cells in the sample. The distribution of cell volumes is plotted on a histogram, and by setting volume thresholds based on the typical sizes of each type of cell, the different cell populations can be identified and counted.[61]

In light scattering techniques, light from a laser or a tungsten-halogen lamp is directed at the stream of cells to collect information about their size and structure. Cells scatter light at different angles as they pass through the beam, which is detected using photometers.[62] Forward scatter, which refers to the amount of light scattered along the beam's axis, is mainly caused by diffraction of light and correlates with cellular size, while side scatter (light scattered at a 90-degree angle) is caused by reflection and refraction and provides information about cellular complexity.[62][63]

Radiofrequency-based methods can be used in combination with impedance. These techniques work on the same principle of measuring the interruption in current as cells pass through an aperture, but since the high-frequency RF current penetrates into the cells, the amplitude of the resulting pulse relates to factors like the relative size of the nucleus, the nucleus's structure, and the amount of granules in the cytoplasm.[64][65] Small red cells and cellular debris, which are similar in size to platelets, may interfere with the platelet count, and large platelets may not be counted accurately, so some analyzers use additional techniques to measure platelets, such as fluorescent staining, multi-angle light scatter and monoclonal antibody tagging.[48]

Most analyzers directly measure the average size of red blood cells, which is called the mean cell volume (MCV), and calculate the hematocrit by multiplying the red blood cell count by the MCV. Some measure the hematocrit by comparing the total volume of red blood cells to the volume of blood sampled, and derive the MCV from the hematocrit and red blood cell count.[66] The hemoglobin concentration, the red blood cell count and the hematocrit are used to calculate the average amount of hemoglobin within each red blood cell, the mean corpuscular hemoglobin (MCH); and its concentration, the mean corpuscular hemoglobin concentration (MCHC).[67] Another calculation, the red blood cell distribution width (RDW), is derived from the standard deviation of the mean cell volume and reflects variation in cellular size.[68]

A scatter plot displaying many differently coloured clusters, labelled with the type of white blood cell they correspond to.
Example of a white blood cell differential scattergram: differently coloured clusters indicate different cell populations

After being treated with reagents, white blood cells form three distinct peaks when their volumes are plotted on a histogram. These peaks correspond roughly to populations of granulocytes, lymphocytes, and other mononuclear cells, allowing a three-part differential to be performed based on cell volume alone.[69][70] More advanced analyzers use additional techniques to provide a five- to seven-part differential, such as light scattering or radiofrequency analysis,[70] or using dyes to stain specific chemicals inside cells—for example, nucleic acids, which are found in higher concentrations in immature cells[71] or myeloperoxidase, an enzyme found in cells of the myeloid lineage.[72][73]Basophils may be counted in a separate channel where a reagent destroys other white cells and leaves basophils intact. The data collected from these measurements is analyzed and plotted on a scattergram, where it forms clusters that correlate with each white blood cell type.[70][72] Another approach to automating the differential count is the use of digital microscopy software,[74] which uses artificial intelligence to classify white blood cells from photomicrographs of the blood smear. The cell images are displayed to a human operator, who can manually re-classify the cells if necessary.[75]

Most analyzers take less than a minute to run all the tests in the complete blood count.[58] Because analyzers sample and count many individual cells, the results are very precise.[76] However, some abnormal cells may not be identified correctly, requiring manual review of the instrument's results and identification by other means of abnormal cells the instrument could not categorize.[5][77]

Point-of-care testing[edit]

Point-of-care testing refers to tests conducted outside of the laboratory setting, such as at a person's bedside or in a clinic.[78][79] This method of testing is faster and uses less blood than conventional methods, and does not require specially trained personnel, so it is useful in emergency situations and in areas with limited access to resources. Commonly used devices for point-of-care hematology testing include the HemoCue, a portable analyzer that uses spectrophotometry to measure the hemoglobin concentration of the sample, and the i-STAT, which derives a hemoglobin reading by estimating the concentration of red blood cells from the conductivity of the blood.[79] Hemoglobin and hematocrit can be measured on point-of-care devices designed for blood gas testing, but these measurements sometimes correlate poorly with those obtained through standard methods.[78] There are simplified versions of hematology analyzers designed for use in clinics that can provide a complete blood count and differential.[80]


Diagram of the manual hematocrit test showing the fraction of red blood cells measured as 0.46.
Manual determination of hematocrit. The blood has been centrifuged, separating it into red blood cells and plasma.

The tests can be performed manually when automated equipment is not available or when the analyzer results indicate that further investigation is needed.[45] Automated results are flagged for manual blood smear review in 10–25% of cases, which may be due to abnormal cell populations that the analyzer cannot properly count,[5] internal flags generated by the analyzer that suggest the results could be inaccurate,[81] or numerical results that fall outside set thresholds.[77] To investigate these issues, blood is spread on a microscope slide, stained with a Romanowsky stain, and examined under a microscope.[82] The appearance of the red and white blood cells and platelets is assessed, and qualitative abnormalities are reported if present.[83] Changes in the appearance of red blood cells can have considerable diagnostic significance—for example, the presence of sickle cells is indicative of sickle cell disease, and a high number of fragmented red blood cells (schistocytes) requires urgent investigation as it can suggest a microangiopathic hemolytic anemia.[84] In some inflammatory conditions and in paraprotein disorders like multiple myeloma, high levels of protein in the blood may cause red blood cells to appear stacked together on the smear, which is termed rouleaux.[85] Some parasitic diseases, such as malaria and babesiosis, can be detected by finding the causative organisms on the blood smear,[86] and the platelet count can be estimated from the blood smear, which is useful if the automated platelet count is inaccurate.[77]

To perform a manual white blood cell differential, the microscopist counts 100 cells on the blood smear and classifies them based on their appearance; sometimes 200 cells are counted.[87] This gives the percentage of each type of white blood cell, and by multiplying these percentages by the total number of white blood cells, the absolute number of each type of white cell can be obtained.[88] Manual counting is subject to sampling error because so few cells are counted compared with automated analysis,[76] but it can identify abnormal cells that analyzers cannot,[72][77] such as the blast cells seen in acute leukemia.[89] Clinically significant features like toxic granulation and vacuolation can also be ascertained from microscopic examination of white blood cells.[90]

The hematocrit can performed manually by filling a capillary tube with blood, centrifuging it, and measuring the percentage of the blood that consists of red blood cells.[66] This is useful in some conditions that can cause automated hematocrit results to be incorrect, such as polycythemia (a highly elevated red blood cell count)[66] or severe leukocytosis (a highly elevated white blood cell count, which interferes with red blood cell measurements by causing white blood cells to be counted as red cells).[91]

=A glass slide containing two chambers to hold fluid, topped with a coverslip
A microscopic image showing numerous cells overlaid on a grid

Left: A modified Fuchs-Rosenthal hemocytometer. Right: View through the microscope of the hemocytometer. The built-in grid helps to keep track of which cells have been counted.

Red and white blood cells and platelets can be counted using a hemocytometer, a microscope slide containing a chamber that holds a specified volume of diluted blood. The hemocytometer's chamber is etched with a calibrated grid to aid in cell counting. The cells seen in the grid are counted and divided by the volume of blood examined, which is determined from the number of squares counted on the grid, to obtain the concentration of cells in the sample.[45][92] Manual cell counts are labour-intensive and inaccurate compared to automated methods, so they are rarely used except in laboratories that do not have access to automated analyzers.[45][92] To count white blood cells, the sample is diluted using a fluid containing a compound that lyses red blood cells, such as ammonium oxalate, acetic acid, or hydrochloric acid.[93] Sometimes a stain is added to the diluent that highlights the nuclei of white blood cells, making them easier to identify. Manual platelet counts are performed in a similar manner, although some methods leave the red blood cells intact. Using a phase-contrast microscope, rather than a light microscope, can make platelets easier to identify.[94] The manual red blood cell count is rarely performed, as it is inaccurate and other methods such as hemoglobinometry and the manual hematocrit are available for assessing red blood cells; but if it is necessary to do so, red blood cells can be counted in blood that has been diluted with saline.[95]

Hemoglobin can be measured manually using a spectrophotometer or colorimeter. To measure hemoglobin manually, the sample is diluted using reagents that destroy red blood cells to release the hemoglobin. Other chemicals are used to convert different types of hemoglobin to one form, allowing it to be easily measured. The solution is then placed in a measuring cuvette and the absorbance is measured at a specific wavelength, which depends on the type of reagent used. A reference standard containing a known amount of hemoglobin is used to determine the relationship between the absorbance and the hemoglobin concentration, allowing the hemoglobin level of the sample to be measured.[96]

In rural and economically disadvantaged areas, available testing is limited by access to equipment and personnel. At primary care facilities in these regions, testing may be limited to examination of red cell morphology and manual measurement of hemoglobin, while more complex techniques like manual cell counts and differentials, and sometimes automated cell counts, are performed at district laboratories. Regional and provincial hospitals and academic centres typically have access to automated analyzers. Where laboratory facilities are not available, an estimate of hemoglobin concentration can be obtained by placing a drop of blood on a standardized type of absorbent paper and comparing it to a colour scale.[97]

Quality control[edit]

Further information: Laboratory quality control

Automated analyzers have to be regularly calibrated. Most manufacturers provide preserved blood with defined parameters and the analyzers are adjusted if the results are outside defined thresholds.[98] To ensure that results continue to be accurate, quality control samples, which are typically provided by the instrument manufacturer, are tested at least once per day. The samples are formulated to provide specific results, and laboratories compare their results against the known values to ensure the instrument is functioning properly.[99][100] For laboratories without access to commercial quality control material, an Indian regulatory organization recommends running patient samples in duplicate and comparing the results.[101] A moving average measurement, in which the average results for patient samples are measured at set intervals, can be used as an additional quality control technique. Assuming that the characteristics of the patient population remain roughly the same over time, the average should remain constant; large shifts in the average value can indicate instrument problems.[99][100] The MCHC values are particularly useful in this regard.[102]

In addition to analyzing internal quality control samples with known results, laboratories may receive external quality assessment samples from regulatory organizations. While the purpose of internal quality control is to ensure that analyzer results are reproducible within a given laboratory, external quality assessment verifies that results from different laboratories are consistent with each other and with the target values.[103] The expected results for external quality assessment samples are not disclosed to the laboratory.[104] External quality assessment programs have been widely adopted in North America and western Europe,[99] and laboratories are often required to participate in these programs to maintain accreditation.[105] Logistical issues may make it difficult for laboratories in under-resourced areas to implement external quality assessment schemes.[106]

Included tests[edit]

The CBC measures the amounts of platelets and red and white blood cells, along with the hemoglobin and hematocrit values. Red blood cell indices—MCV, MCH and MCHC—which describe the size of red blood cells and their hemoglobin content, are reported along with the red blood cell distribution width (RDW), which measures the amount of variation in the sizes of red blood cells. A white blood cell differential, which enumerates the different types of white blood cells, may be performed, and a count of immature red blood cells (reticulocytes) is sometimes included.[4][107]

Red blood cells, hemoglobin, and hematocrit[edit]

Main articles: Red blood cell, hemoglobin, hematocrit, and red blood cell indices

Analyte Result Normal range
Red cell count 5.5 x 1012/L4.5–5.7
White cell count 9.8 x 109/L4.0–10.0
Hemoglobin 123 g/L133–167
Hematocrit 0.420.35–0.53
MCV 76 fL77–98
MCH 22.4 pg26–33
MCHC 293 g/L330–370
RDW 14.5%10.3–15.3

An example of CBC results showing a low hemoglobin, mean red cell volume (MCV), mean red cell hemoglobin (MCH) and mean red blood cell hemoglobin content (MCHC). The person was anemic. The cause could be iron deficiency or a hemoglobinopathy.[108]

Red blood cells deliver oxygen from the lungs to the tissues and on their return carry carbon dioxide back to the lungs where it is exhaled. These functions are mediated by the cells' hemoglobin.[109] The analyzer counts red blood cells, reporting the result in units of 106 cells per microlitre of blood (× 106/μL) or 1012 cells per litre (× 1012/L), and measures their average size, which is called the mean cell volume and expressed in femtolitres or cubic micrometres.[4] By multiplying the mean cell volume by the red blood cell count, the hematocrit (HCT) or packed cell volume (PCV), a measurement of the percentage of blood that is made up of red blood cells, can be derived;[66] and when the hematocrit is performed directly, the mean cell volume may be calculated from the hematocrit and red blood cell count.[110][111] Hemoglobin, measured after the red blood cells are lysed, is usually reported in units of grams per litre (g/L) or grams per decilitre (g/dL).[112] Assuming that the red blood cells are normal, there is a constant relationship between hemoglobin and hematocrit: the hematocrit percentage is approximately three times greater than the hemoglobin value in g/dL, plus or minus three. This relationship, called the rule of three, can be used to confirm that CBC results are correct.[113]

Two other measurements are calculated from the red blood cell count, the hemoglobin concentration, and the hematocrit: the mean corpuscular hemoglobin and the mean corpuscular hemoglobin concentration.[114][115] These parameters describe the hemoglobin content of each red blood cell. The MCH and MCHC can be confusing; in essence the MCH is a measure of the average amount of hemoglobin per red blood cell. The MCHC gives the average proportion of the cell that is hemoglobin. The MCH does not take into account the size of the red blood cells whereas the MCHC does.[116] Collectively, the MCV, MCH, and MCHC are referred to as the red blood cell indices.[114][115] Changes in these indices are visible on the blood smear: red blood cells that are abnormally large or small can be identified by comparison to the sizes of white blood cells, and cells with a low hemoglobin concentration appear pale.[117] Another parameter is calculated from the initial measurements of red blood cells: the red blood cell distribution width or RDW, which reflects the degree of variation in the cells' size.[118]

An abnormally low hemoglobin, hematocrit, or red blood cell count indicates anemia.[119] Anemia is not a diagnosis on its own, but it points to an underlying condition affecting the person's red blood cells.[88] General causes of anemia include blood loss, production of defective red blood cells (ineffective erythropoeisis), decreased production of red blood cells (insufficient erythropoeisis), and increased destruction of red blood cells (hemolytic anemia).[120] Anemia reduces the blood's ability to carry oxygen, causing symptoms like tiredness and shortness of breath.[121] If the hemoglobin level falls below thresholds based on the person's clinical condition, a blood transfusion may be necessary.[122]

An increased number of red blood cells, which usually leads to an increase in the hemoglobin and hematocrit,[note 4] is called polycythemia.[126]Dehydration or use of diuretics can cause a "relative" polycythemia by decreasing the amount of plasma compared to red cells. A true increase in the number of red blood cells, called absolute polycythemia, can occur when the body produces more red blood cells to compensate for chronically low oxygen levels in conditions like lung or heart disease, or when a person has abnormally high levels of erythropoietin (EPO), a hormone that stimulates production of red blood cells. In polycythemia vera, the bone marrow produces red cells and other blood cells at an excessively high rate.[127]

Evaluation of red blood cell indices is helpful in determining the cause of anemia. If the MCV is low, the anemia is termed microcytic, while anemia with a high MCV is called macrocytic anemia. Anemia with a low MCHC is called hypochromic anemia. If anemia is present but the red blood cell indices are normal, the anemia is considered normochromic and normocytic.[117] The term hyperchromia, referring to a high MCHC, is generally not used. Elevation of the MCHC above the upper reference value is rare, mainly occurring in conditions such as spherocytosis, sickle cell disease and hemoglobin C disease.[115][128] An elevated MCHC can also be a false result from conditions like red blood cell agglutination (which causes a false decrease in the red blood cell count, elevating the MCHC)[129][130] or highly elevated amounts of lipids in the blood (which causes a false increase in the hemoglobin result).[128][131]

Microcytic anemia is typically associated with iron deficiency, thalassemia, and anemia of chronic disease, while macrocytic anemia is associated with alcoholism, folate and B12 deficiency, use of some drugs, and some bone marrow diseases. Acute blood loss, hemolytic anemia, bone marrow disorders, and various chronic diseases can result in anemia with a normocytic blood picture.[115][132] The MCV serves an additional purpose in laboratory quality control. It is relatively stable over time compared to other CBC parameters, so a large change in MCV may indicate that the sample was drawn from the wrong patient.[133]

A low RDW has no clinical significance, but an elevated RDW represents increased variation in red blood cell size, a condition known as anisocytosis.[118] Anisocytosis is common in nutritional anemias such as iron deficiency anemia and anemia due to vitamin B12 or folate deficiency, while people with thalassemia may have a normal RDW.[118] Based on the CBC results, further steps can be taken to investigate anemia, such as a ferritin test to confirm the presence of iron deficiency, or hemoglobin electrophoresis to diagnose a hemoglobinopathy such as thalassemia or sickle cell disease.[134]

White blood cells[edit]

Main articles: White blood cell and white blood cell differential

Analyte Result
White cell count 98.8 x 109/L
Hemoglobin 116 g/L
Hematocrit 0.349 L/L
MCV 89.0 fL
Platelet count 1070 x 109/L
Analyte Result
Neutrophils 48%
Lymphocytes 3%
Monocytes 4%
Eosinophils 3%
Basophils 21%
Band neutrophils 8%
Metamyelocytes 3%
Myelocytes 8%
Blast cells 2%

The white blood cell and platelet counts are markedly increased, and anemia is present. The differential count shows basophilia and the presence of band neutrophils, immature granulocytes and blast cells.[135]

White blood cells defend against infections and are involved in the inflammatory response.[136] A high white blood cell count, which is called leukocytosis, often occurs in infections, inflammation, and states of physiologic stress. It can also be caused by diseases that involve abnormal production of blood cells, such as myeloproliferative and lymphoproliferative disorders.[137] A decreased white blood cell count, termed leukopenia, can lead to an increased risk of acquiring infections,[138] and occurs in treatments like chemotherapy and radiation therapy and many conditions that inhibit the production of blood cells.[139] Sepsis is associated with both leukocytosis and leukopenia.[140] The total white blood cell count is usually reported in cells per microlitre of blood (/μL) or 109 cells per litre (× 109/L).[4]

In the white blood cell differential, the different types of white blood cells are identified and counted. The results are reported as a percentage and as an absolute number per unit volume. Five types of white blood cells—neutrophils, lymphocytes, monocytes, eosinophils, and basophils—are typically measured.[141] Some instruments report the number of immature granulocytes, which is a classification consisting of precursors of neutrophils; specifically, promyelocytes, myelocytes and metamyelocytes.[note 5][144] Other cell types are reported if they are identified in the manual differential.[145]

Differential results are useful in diagnosing and monitoring many medical conditions. For example, an elevated neutrophil count (neutrophilia) is associated with bacterial infection, inflammation, and myeloproliferative disorders,[146][147] while a decreased count (neutropenia) may occur in individuals who are undergoing chemotherapy or taking certain drugs, or who have diseases affecting the bone marrow.[148][149] Neutropenia can also be caused by some congenital disorders and may occur transiently after viral or bacterial infections in children.[150] People with severe neutropenia and clinical signs of infection are treated with antibiotics to prevent potentially life-threatening disease.[151]

An increased number of band neutrophils—young neutrophils that lack segmented nuclei—or immature granulocytes is termed left shift and occurs in sepsis and some blood disorders, but is normal in pregnancy.[152][153] An elevated lymphocyte count (lymphocytosis) is associated with viral infection[6] and lymphoproliferative disorders like chronic lymphocytic leukemia;[154] elevated monocyte counts (monocytosis) are associated with chronic inflammatory states;[155] and the eosinophil count is often increased (eosinophilia) in parasitic infections and allergic conditions.[156] An increased number of basophils, termed basophilia, can occur in myeloproliferative disorders like chronic myeloid leukemia and polycythemia vera.[147] The presence of some types of abnormal cells, such as blast cells or lymphocytes with neoplastic features, is suggestive of a hematologic malignancy.[89][157]


Main articles: Platelet and mean platelet volume

Platelets play an essential role in clotting. When the wall of a blood vessel is damaged, platelets adhere to the exposed surface at the site of injury and plug the gap. Simultaneous activation of the coagulation cascade results in the formation of fibrin, which reinforces the platelet plug to create a stable clot.[158] A low platelet count, known as thrombocytopenia, may cause bleeding if severe.[159] It can occur in individuals who are undergoing treatments that suppress the bone marrow, such as chemotherapy or radiation therapy, or taking certain drugs, such as heparin, that can induce the immune system to destroy platelets. Thrombocytopenia is a feature of many blood disorders, like acute leukemia and aplastic anemia, as well as some autoimmune diseases.[160][161] If the platelet count is extremely low, a platelet transfusion may be performed.[162]Thrombocytosis, meaning a high platelet count, may occur in states of inflammation or trauma,[163] as well as in iron deficiency,[164] and the platelet count may reach exceptionally high levels in people with essential thrombocythemia, a rare blood disease.[163] The platelet count can be reported in units of cells per microlitre of blood (/μL),[165] 103 cells per microlitre (× 103/μL), or 109 cells per litre (× 109/L).[4]

The mean platelet volume (MPV) measures the average size of platelets in femtolitres. It can aid in determining the cause of thrombocytopenia; an elevated MPV may occur when young platelets are released into the bloodstream to compensate for increased destruction of platelets, while decreased production of platelets due to dysfunction of the bone marrow can result in a low MPV. The MPV is also useful for differentiating between congenital diseases that cause thrombocytopenia.[118][166] The immature platelet fraction (IPF) or reticulated platelet count is reported by some analyzers and provides information about the rate of platelet production by measuring the number of immature platelets in the blood.[167]

Other tests[edit]

Reticulocyte count[edit]

Main article: Reticulocyte

Microscopic image of red blood cells stained blue.
Red blood cells stained with new methylene blue: the cells containing dark blue structures are reticulocytes.

Reticulocytes are immature red blood cells, which, unlike the mature cells, contain RNA. A reticulocyte count is sometimes performed as part of a complete blood count, usually to investigate the cause of a person's anemia or evaluate their response to treatment. Anemia with a high reticulocyte count can indicate that the bone marrow is producing red blood cells at a higher rate to compensate for blood loss or hemolysis,[74] while anemia with a low reticulocyte count may suggest that the person has a condition that reduces the body's ability to produce red blood cells.[168] When people with nutritional anemia are given nutrient supplementation, an increase in the reticulocyte count indicates that their body is responding to the treatment by producing more red blood cells.[169] Hematology analyzers perform reticulocyte counts by staining red blood cells with a dye that binds to RNA and measuring the number of reticulocytes through light scattering or fluorescence analysis. The test can be performed manually by staining the blood with new methylene blue and counting the percentage of red blood cells containing RNA under the microscope. The reticulocyte count is expressed as an absolute number[168] or as a percentage of red blood cells.[170]

Some instruments measure the average amount of hemoglobin in each reticulocyte; a parameter that has been studied as an indicator of iron deficiency in people who have conditions that interfere with standard tests.[171] The immature reticulocyte fraction (IRF) is another measurement produced by some analyzers which quantifies the maturity of reticulocytes: cells that are less mature contain more RNA and thus produce a stronger fluorescent signal. This information can be useful in diagnosing anemias and evaluating red blood cell production following anemia treatment or bone marrow transplantation.[172]

Nucleated red blood cells[edit]

Main article: Nucleated red blood cell

During their formation in bone marrow, and in the liver and spleen in fetuses,[173] red blood cells contain a cell nucleus, which is usually absent in the mature cells that circulate in the bloodstream.[174] When detected, the presence of nucleated red cells, particularly in children and adults, indicates an increased demand for red blood cells, which can be caused by bleeding, some cancers and anemia.[118] Most analyzers can detect these cells as part of the differential cell count. High numbers of nucleated red cells can cause a falsely high white cell count, which will require adjusting.[175]

Other parameters[edit]

Advanced hematology analyzers generate novel measurements of blood cells which have shown diagnostic significance in research studies but have not yet found widespread clinical use.[171] For example, some types of analyzers produce coordinate readings indicating the size and position of each white blood cell cluster. These parameters (termed cell population data)[176] have been studied as potential markers for blood disorders, bacterial infections and malaria. Analyzers that use myeloperoxidase staining to produce differential counts can measure white blood cells' expression of the enzyme, which is altered in various disorders.[75] Some instruments can report the percentage of red blood cells that are hypochromic in addition to reporting the average MCHC value, or provide a count of fragmented red cells (schistocytes),[171] which occur in some types of hemolytic anemia.[177] Because these parameters are often specific to particular brands of analyzers, it is difficult for laboratories to interpret and compare results.[171]

Reference ranges[edit]

See also: Reference ranges for blood tests

The complete blood count is interpreted by comparing the output to reference ranges, which represent the results found in 95% of apparently healthy people.[35] Based on a statistical normal distribution, the tested samples' ranges vary with gender and age. On average, adult females have lower hemoglobin, hematocrit, and red blood cell count values than males; the difference lessens, but is still present, after menopause.[179]

The blood of newborn babies is very different from that of older children, which is different again from the blood of adults. Newborns' hemoglobin, hematocrit, and red blood cell count are extremely high to compensate for low oxygen levels in the womb, and a high proportion of fetal hemoglobin, which is less effective at delivering oxygen to tissues than mature forms of hemoglobin, inside their red blood cells.[180][181] The MCV is also increased, and the white blood cell count is elevated with a preponderance of neutrophils.[180][182] The red blood cell count and related values begin to decline shortly after birth, reaching their lowest point at about two months of age and increasing thereafter.[183][184] The red blood cells of older infants and children are smaller, with a lower MCH, than those of adults. In the pediatric white blood cell differential, lymphocytes often outnumber neutrophils, while in adults neutrophils predominate.[180]

Other differences between populations may affect the reference ranges: for example, people living at higher altitudes have higher hemoglobin, hematocrit, and RBC results, and people of African heritage have lower white blood cell counts on average.[185] The type of analyzer used to run the CBC affects the reference ranges as well. Reference ranges are therefore established by individual laboratories based on their own patient populations and equipment.[186][187]


Some medical conditions or problems with the blood sample may produce inaccurate results. If the sample is visibly clotted, which can be caused by poor phlebotomy technique, it is unsuitable for testing, because the platelet count will be falsely decreased and other results may be abnormal.[188][189] Samples stored at room temperature for several hours may give falsely high readings for MCV,[190] because red blood cells swell as they absorb water from the plasma; and platelet and white blood cell differential results may be inaccurate in aged specimens, as the cells degrade over time.[91]

Samples drawn from individuals with very high levels of bilirubin or lipids in their plasma (referred to as an icteric sample or a lipemic sample, respectively)[191] may show falsely high readings for hemoglobin, because these substances change the colour and opacity of the sample, which interferes with hemoglobin measurement.[192] This effect can be mitigated by replacing the plasma with saline.[91]

Some individuals produce an antibody that causes their platelets to form clumps when their blood is drawn into tubes containing EDTA, the anticoagulant typically used to collect CBC samples. Platelet clumps may be counted as single platelets by automated analyzers, leading to a falsely decreased platelet count. This can be avoided by using an alternative anticoagulant such as sodium citrate or heparin.[193]

Another antibody-mediated condition that can affect complete blood count results is red blood cell agglutination. This phenomenon causes red blood cells to clump together because of antibodies bound to the cell surface.[194] Red blood cell aggregates are counted as single cells by the analyzer, leading to a markedly decreased red blood cell count and hematocrit, and markedly elevated MCV and MCHC.[53] Often, these antibodies are only active at room temperature (in which case they are called cold agglutinins), and the agglutination can be reversed by heating the sample to 37 °C (99 °F). Samples from people with warm autoimmune hemolytic anemia may exhibit red cell agglutination that does not resolve on warming.[130]

While blast and lymphoma cells can be identified in the manual differential, microscopic examination cannot reliably determine the cells' hematopoietic lineage. This information is often necessary for diagnosing blood cancers. After abnormal cells are identified, additional techniques such as immunophenotyping by flow cytometry can be used to identify markers that provide additional information about the cells.[195][196]


A black leather case with its contents: a candle and colour cards
An early hemoglobinometer: blood samples were compared to a colour chart of reference standards to determine the hemoglobin level.[197]

Before automated cell counters were introduced, complete blood count tests were performed manually: white and red blood cells and platelets were counted using microscopes.[198] The first person to publish microscopic observations of blood cells was Antonie van Leeuwenhoek,[199] who reported on the appearance of red cells in a 1674 letter to the Proceedings of the Royal Society of London.[200]Jan Swammerdam had described red blood cells some years earlier, but did not publish his findings at the time. Throughout the 18th and 19th centuries, improvements in microscope technology such as achromatic lenses allowed white blood cells and platelets to be counted in unstained samples.[201]

The physiologist Karl Vierordt is credited with performing the first blood count.[8][202][203] His technique, published in 1852, involved aspirating a carefully measured volume of blood into a capillary tube and spreading it onto a microscope slide coated with egg white. After the blood dried, he counted every cell on the slide; this process could take more than three hours to complete.[204] The hemocytometer, introduced in 1874 by Louis-Charles Malassez, simplified the microscopic counting of blood cells.[205] Malassez's hemocytometer consisted of a microscope slide containing a flattened capillary tube. Diluted blood was introduced to the capillary chamber by means of a rubber tube attached to one end, and an eyepiece with a scaled grid was attached to the microscope, permitting the microscopist to count the number of cells per volume of blood. In 1877, William Gowers invented a hemocytometer with a built-in counting grid, eliminating the need to produce specially calibrated eyepieces for each microscope.[206]

In the 1870s, Paul Ehrlich developed a staining technique using a combination of an acidic and basic dye that could distinguish different types of white blood cells and allow red blood cell morphology to be examined.[201]Dmitri Leonidovich Romanowsky improved on this technique in the 1890s, using a mixture of eosin and aged methylene blue to produce a wide range of hues not present when either of the stains was used alone. This became the basis for Romanowsky staining, the technique still used to stain blood smears for manual review.[207]

The first techniques for measuring hemoglobin were devised in the late 19th century, and involved visual comparisons of the colour of diluted blood against a known standard.[203] Attempts to automate this process using spectrophotometry and colorimetry were limited by the fact that hemoglobin is present in the blood in many different forms, meaning that it could not be measured at a single wavelength. In 1920, a method to convert the different forms of hemoglobin to one stable form (cyanmethemoglobin or hemiglobincyanide) was introduced, allowing hemoglobin levels to be measured automatically. The cyanmethemoglobin method remains the reference method for hemoglobin measurement and is still used in many automated hematology analyzers.[57][208][209]

Maxwell Wintrobe is credited with the invention of the hematocrit test.[66][210] In 1929, he undertook a PhD project at the University of Tulane to determine normal ranges for red blood cell parameters, and invented a method known as the Wintrobe hematocrit. Hematocrit measurements had previously been described in the literature, but Wintrobe's method differed in that it used a large tube that could be mass-produced to precise specifications, with a built-in scale. The fraction of red blood cells in the tube was measured after centrifugation to determine the hematocrit. The invention of a reproducible method for determining hematocrit values allowed Wintrobe to define the red blood cell indices.[203]

A complex tube and flask apparatus attached to a measurement station

Research into automated cell counting began in the early 20th century.[209] A method developed in 1928 used the amount of light transmitted through a diluted blood sample, as measured by photometry, to estimate the red blood cell count, but this proved inaccurate for samples with abnormal red blood cells.[8] Other unsuccessful attempts, in the 1930s and 1940s, involved photoelectric detectors attached to microscopes, which would count cells as they were scanned.[209] In the late 1940s, Wallace H. Coulter, motivated by a need for better red blood cell counting methods following the bombing of Hiroshima and Nagasaki,[211] attempted to improve on photoelectric cell counting techniques.[note 7] His research was aided by his brother, Joseph R. Coulter, in a basement laboratory in Chicago.[60] Their results using photoelectric methods were disappointing, and in 1948, after reading a paper relating the conductivity of blood to its red blood cell concentration, Wallace devised the Coulter principle—the theory that a cell suspended in a conductive medium generates a drop in current proportional to its size as it passes through an aperture.[211]

That October, Wallace built a counter to demonstrate the principle. Owing to financial constraints, the aperture was made by burning a hole through a piece of cellophane from a cigarette package.[60][211] Wallace filed a patent for the technique in 1949, and in 1951 applied to the Office of Naval Research to fund the development of the Coulter counter.[211] Wallace's patent application was granted in 1953, and after improvements to the aperture and the introduction of a mercury manometer to provide precise control over sample size, the brothers founded Coulter Electronics Inc. in 1958 to market their instruments. The Coulter counter was initially designed for counting red blood cells, but with later modifications it proved effective for counting white blood cells.[60] Coulter counters were widely adopted by medical laboratories.[209]

The first analyzer able to produce multiple cell counts simultaneously was the TechniconSMA 4A−7A, released in 1965. It achieved this by partitioning blood samples into two channels: one for counting red and white blood cells and one for measuring hemoglobin. However, the instrument was unreliable and difficult to maintain. In 1968, the Coulter Model S analyzer was released and gained widespread use. Similarly to the Technicon instrument, it used two different reaction chambers, one of which was used for the red cell count, and one of which was used for the white blood cell count and hemoglobin determination. The Model S also determined the mean cell volume using impedance measurements, which allowed the red blood cell indices and hematocrit to be derived. Automated platelet counts were introduced in 1970 with Technicon's Hemalog-8 instrument and were adopted by Coulter's S Plus series analyzers in 1980.[212]

After basic cell counting had been automated, the white blood cell differential remained a challenge. Throughout the 1970s, researchers explored two methods for automating the differential count: digital image processing and flow cytometry. Using technology developed in the 1950s and 60s to automate the reading of Pap smears, several models of image processing analyzers were produced.[213] These instruments would scan a stained blood smear to find cell nuclei, then take a higher resolution snapshot of the cell to analyze it through densitometry.[214] They were expensive, slow, and did little to reduce workload in the laboratory because they still required blood smears to be prepared and stained, so flow cytometry-based systems became more popular,[215][216] and by 1990, no digital image analyzers were commercially available in the United States or western Europe.[217] These techniques enjoyed a resurgence in the 2000s with the introduction of more advanced image analysis platforms using artificial neural networks.[218][219][220]

Early flow cytometry devices shot beams of light at cells in specific wavelengths and measured the resulting absorbance, fluorescence or light scatter, collecting information about the cells' features and allowing cellular contents such as DNA to be quantified.[221] One such instrument—the Rapid Cell Spectrophotometer, developed by Louis Kamentsky in 1965 to automate cervical cytology—could generate blood cell scattergrams using cytochemical staining techniques. Leonard Ornstein, who had helped to develop the staining system on the Rapid Cell Spectrophotometer, and his colleagues later created the first commercial flow cytometric white blood cell differential analyzer, the Hemalog D.[222][223] Introduced in 1974,[224][225] this analyzer used light scattering, absorbance and cell staining to identify the five normal white blood cell types in addition to "large unidentified cells", a classification that usually consisted of atypical lymphocytes or blast cells. The Hemalog D could count 10,000 cells in one run, a marked improvement over the manual differential.[223][226] In 1981, Technicon combined the Hemalog D with the Hemalog-8 analyzer to produce the Technicon H6000, the first combined complete blood count and differential analyzer. This analyzer was unpopular with hematology laboratories because it was labour-intensive to operate, but in the late 1980s to early 1990s similar systems were widely produced by other manufacturers such as Sysmex, Abbott, Roche and Beckman Coulter.[227]

Explanatory notes[edit]

  1. ^Though commonly referred to as such, platelets are technically not cells: they are cell fragments, formed from the cytoplasm of megakaryocytes in the bone marrow.[6]
  2. ^The data used to construct reference ranges is usually derived from "normal" subjects, but it is possible for these individuals to have asymptomatic disease.[34]
  3. ^In its broadest sense, the term flow cytometry refers to any measurement of the properties of individual cells in a fluid stream,[49][50] and in this respect, all hematology analyzers (except those using digital image processing) are flow cytometers. However, the term is commonly used in reference to light scattering and fluorescence methods, especially those involving the identification of cells using labelled antibodies that bind to cell surface markers (immunophenotyping).[49][51]
  4. ^This is not always the case. In some types of thalassemia, for example, a high red blood cell count occurs alongside a low or normal hemoglobin, as the red blood cells are very small.[123][124] The Mentzer index, which compares the MCV to the RBC count, can be used to distinguish between iron deficiency anemia and thalassemia.[125]
  5. ^Automated instruments group these three types of cells together under the "immature granulocyte" classification,[142] but they are counted separately in the manual differential.[143]
  6. ^The RDW is highly elevated at birth and gradually decreases until approximately six months of age.[178]
  7. ^An apocryphal story holds that Wallace invented the Coulter counter to study particles in paints used on US Navy ships; other accounts claim it was originally designed during the Second World War to count plankton. However, Wallace never worked for the Navy, and his earliest writings on the device state that it was first used to analyze blood. The paint story was eventually retracted from documents produced by the Wallace H. Coulter Foundation.[211]



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