The complete blood count (CBC) is a screening test, used to diagnose and manage numerous diseases. It can reflect problems with fluid volume (such as dehydration) or loss of blood. It can show abnormalities in the production, life span, and destruction of blood cells. It can reflect acute or chronic infection, allergies, and problems with clotting.
The CBC test identifies and counts the 7 types of cells found in the blood: neutrophil, eosinophil, basophil, red blood cell, lymphocyte, monocyte, and platelet.
Procedure
A CBC requires a small blood specimen. Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand.
Preparation: The skin should be cleaned with alcohol or iodine before the test. The patient should be seated comfortably or reclining.
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. After the blood is drawn, there may be some throbbing.
Although the CBC test is very safe, any blood drawing has a very small risk of complication, including:
Oozing of blood from one puncture site
Fainting or feeling lightheaded
Hematoma (blood accumulating under the skin)
Multiple punctures to locate veins
Infection (a slight risk any time the skin is broken)
Results, part 1
Results: Normal values vary with altitude and gender.
What abnormal results may mean: Low numbers of red blood cells (hematocrit) may indicate anemia, which has many causes, including:
Blood loss
Iron deficiencies
Deficiencies of vitamin B12 or folic acid
Bone marrow failure (for example, from radiation, toxin, fibrosis, tumor)
Erythropoietin deficiency (secondary to kidney disease)
Hemolysis (RBC destruction)
Leukemia
Multiple myeloma
Over hydration
High numbers of red blood cells may indicate:
Dehydration
Burns
Diarrhea
Eclampsia
Erythrocytosis
Polycythemia vera
Shock
Low numbers of white blood cells (leukopenia) may indicate:
Bone marrow failure (for example, due to granuloma, tumor, or fibrosis)
Presence of cytotoxic substance
Collagen-vascular diseases (such as lupus erythematosus)
Disease of the liver or spleen
Radiation exposure
High numbers of white blood cells (leukocytosis) may indicate:
Infectious diseases
Inflammatory disease (such as rheumatoid arthritis or allergy)
Leukemia
Severe emotional or physical stress
Tissue damage (for example, burns)
Certain medicines (such as prednisone and other steroids)
Results, part 2
High numbers of red blood cells may indicate:
Cigarette smoking
Low oxygen tension in the blood.
Congenital heart disease.
Cor pulmonale.
Pulmonary fibrosis.
Polycythemia vera.
Dehydration (such as from severe diarrhea).
Emphysema
Review Date:
9/2/2009
Reviewed By:
Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.
