Definition of WBC count:

A WBC count is a blood test to measure the number of white blood cells (WBCs).

White blood cells help fight infections. They are also called leukocytes. There are five major types of white blood cells:

• Basophils
• Eosinophils
• Lymphocytes (T cells and B cells)
• Monocytes
• Neutrophils

See also: Blood differential

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.

The blood sample is sent to a laboratory. A WBC count is almost always done as part of a complete blood count (CBC).

How to prepare for the test:

No special preparation is usually needed. Tell your doctor about any medications you are taking, including over-the-counter products. Certain drugs may interfere with test results.

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

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. Afterward, there may be some throbbing.

Why the test is performed:

Your doctor will order this test 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.