Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

A WBC count is almost always done as part of a CBC (complete blood count).

No special preparation is usually needed.

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

The test is performed to find out how many white blood cells you have. Your body produces more white blood cells when you have an infection or allergic reaction -- even when you are under general stress.

4,500-10,000 white blood cells/mcL (cells per microliter)

A low number of WBCs is called leukopenia. It may be due to:

• Bone marrow failure (for example, due to infection, tumor, or abnormal scarring)
• Collagen-vascular diseases (such as lupus erythematosus )
• Disease of the liver or spleen
• Radiation

A high number of WBCs is called leukocytosis. It may be due to:

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

These lists are not all inclusive.

A simple blood test is generally safe. In general, risks may include:

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

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

People who have had their spleen removed (splenectomy ) will always have a slightly higher number of WBCs.

Drugs that may increase WBC counts include:

• Allopurinol
• Aspirin
• Chloroform
• Corticosteroids
• Epinephrine
• Heparin
• Quinine
• Triamterene

Drugs that may lower your WBC count include:

• Antibiotics
• Anticonvulsants
• Antihistamines
• Antithyroid drugs
• Arsenicals
• Barbiturates
• Chemotherapy drugs
• Diuretics
• Sulfonamides

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:461-2.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2676-77, 2733.