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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 isolates and counts the 7 types of cells found in the blood: neutrophil, eosinophil, basophil, red blood cell, lymphocyte, monocyte, and platelet.
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 slight risk of complication, including:
Oozing of blood from 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
Normal values vary with altitude and gender.
What abnormal results may mean:
Low numbers of red blood cells may indicate anemia, which has many causes including:
Deficiences 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)
Low numbers of white blood cells (leukopenia) may indicate:
Bone marrow failure (for example, due to granuloma (granular tumor), tumor, or fibrosis)
Presence of cytotoxic substance
Collagen-vascular diseases (such as lupus erythematosus)
Disease of the liver or spleen
High numbers of white blood cells (leukocytosis) may indicate:
Inflammatory disease (such as rheumatoid arthritis or allergy)
Severe emotional or physical stress
Tissue damage (for example, burns)
A high hematocrit may indicate:
Results, part 2
High numbers of red blood cells may indicate:
Low oxygen tension in the blood
Congenital heart disease
Dehydration (such as from severe diarrhea)
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.