A complete blood count (CBC) test measures the following:
- The number of red blood cells (RBCs)
- The number of white blood cells (WBCs)
- 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
How the test is performed
Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
How to prepare for the test
There is no special preparation needed.
How the test will feel
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.
Why the test is performed
The CBC test may be performed under many different conditions and to assess many different symptoms or diseases. The results can reflect problems with fluid volume (such as dehydration) or loss of blood.
The test can reveal problems with RBC production and destruction, or help diagnose infection, allergies, and problems with blood clotting.
MCV, MCH, and MCHC values reflect the size and hemoglobin concentration of individual cells, and are useful in diagnosing different types of anemia.
- RBC count (varies with altitude):
- 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 (varies with altitude):
- Male: 40.7 to 50.3%
- Female: 36.1 to 44.3%
- Hemoglobin (varies with altitude):
- Male: 13.8 to 17.2 gm/dL
- Female: 12.1 to 15.1 gm/dL
- MCV: 80 to 95 femtoliter
- MCH: 27 to 31 pg/cell
- MCHC: 32 to 36 gm/dL
(cells/mcL = cells per microliter; gm/dL = grams per deciliter; pg/cell = picograms per cell)
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
High numbers of RBCs or a high hematocrit may be due to:
- Dehydration (such as from severe diarrhea)
- Kidney disease with high erythropoietin production
- Low oxygen level in the blood:
- Polycythemia vera
Low numbers of RBCs or low hematocrit indicates anemia, which can result from:
- Autoimmune/collagen-vascular diseases such as lupus erythematosus or rheumatoid arthritis
- Blood loss (hemorrhage)
- Bone marrow failure (for example, from radiation, infection, or tumor)
- Erythropoietin deficiency (usually secondary to kidney disease)
- Hemolysis (red blood cell destruction)
- Malnutrition (nutritional deficiencies of 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/collagen-vascular 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)
- Severe emotional or physical stress
- Tissue damage (such as burns)
Low hemoglobin values may indicate:
- Anemia (various types)
- Blood loss
What the risks are
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)
Newland J. The peripheral blood smear. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 161.
Zuckerman K. Approach to the anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 162.
Reviewed By: A.D.A.M. Editorial Team: David Zieve, MD, MHA, David R. Eltz. Previously reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine (2/9/2010).