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A complete blood count (CBC) test measures the following:

• The number of red blood cells (RBC count)
• The number of white blood cells (WBC count)
• The total amount of hemoglobin in the blood
• The fraction of the blood composed of red blood cells (hematocrit)

The CBC test also provides information about the following measurements:

• Average red blood cell size (MCV)
• Hemoglobin amount per red blood cell (MCH)
• The amount of hemoglobin relative to the size of the cell (hemoglobin concentration) per red blood cell (MCHC)

The platelet count is also usually included in the CBC. 

Complete blood count

A blood sample is needed. For information on how this is done, see: Venipuncture

There is no special preparation needed.

When the needle is inserted to draw blood, you may feel moderate pain, though most people feel only a prick or a stinging sensation. Afterward there may be some throbbing or bruising.

A complete blood count (CBC) is used to detect or monitor many different health conditions. It may be used to:

• Diagnose infections or allergies
• Detect blood clotting problems or blood disorders, including anemia
• Evaluate red blood cell production or destruction

Blood counts may vary with altitude. In general, normal results are:

RBC count:

• Male: 4.7 to 6.1 million cells/mcL
• Female: 4.2 to 5.4 million cells/mcL

WBC count:

• 4,500 to 10,000 cells/mcL

Hematocrit:

• Male: 40.7 to 50.3%
• Female: 36.1 to 44.3%

Hemoglobin:

• Male: 13.8 to 17.2 gm/dL
• Female: 12.1 to 15.1 gm/dL

Red blood cell indices:

• MCV: 80 to 95 femtoliter
• MCH: 27 to 31 pg/cell
• MCHC: 32 to 36 gm/dL

Note:

• cells/mcL = cells per microliter
• gm/dL = grams per deciliter;
• pg/cell = picograms per cell

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

A high RBC or hematocrit may be due to:

• Dehydration (such as from severe diarrhea)
• Kidney disease with high erythropoietin production
• Low oxygen level in the blood for a long time due to heart or lung disease
• Polycythemia vera
• Smoking

A low RBC or hematacrit is a sign of anemia, which can result from:

• Autoimmune diseases such as lupus erythematosus or rheumatoid arthritis
• Blood loss (hemorrhage)
• Bone marrow failure (for example, from radiation, infection, or tumor)
• Chronic kidney disease
• Hemolysis (red blood cell destruction)
• Leukemia and other blood cancers
• Long-term infections such as hepatitis
• Poor diet and nutrition, causing too little iron, folate, vitamin B12, or vitamin B6
• Multiple myeloma

A lower than normal white blood cell count is called leukopenia. A decreased WBC count may be due to:

• Autoimmune diseases (such as systemic lupus erythematosus)
• Bone marrow failure (for example, due to infection, tumor, radiation, or fibrosis)
• Disease of the liver or spleen

High numbers of WBCs is called leukocytosis. It can result from:

• Infectious diseases
• Inflammatory disease (such as rheumatoid arthritis or allergy)
• Leukemia
• Severe emotional or physical stress
• Tissue damage (such as burns)

Low hemoglobin values may be due to:

• Anemia (various types)
• Blood loss

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

RBCs transport hemoglobin which, in turn, carries oxygen. The amount of oxygen received by body tissues depends on the amount and function of RBCs and hemoglobin.

WBCs are mediators of inflammation and the immune response. There are various types of WBCs that normally appear in the blood:

• Neutrophils (polymorphonuclear leukocytes)
• Band cells (slightly immature neutrophils)
• T-type lymphocytes (T cells)
• B-type lymphocytes (B cells)
• Monocytes
• Eosinophils
• Basophils

Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 161.