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:

Normal values vary with altitude and sex.

What abnormal results may mean:

Low numbers of red blood cells may indicate anemia, which has many causes including:

• Blood loss
• Iron deficiency
• 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)
• Leukemia
• Multiple myeloma
• Over hydration

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
• 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)

A high hematocrit may indicate:

• Dehydration
• Burns
• Diarrhea
• Eclampsia
• Erythrocytosis
• Polycythemia vera
• Shock

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