A WBC count measures the number of white blood cells in a sample of blood. It is a valuable diagnostic tool for a number of diseases that is usually ordered as part of a complete blood count (CBC).

Who is a candidate for the test?

This test is normally performed to detect an infection or inflammation. A healthcare provider may also use this test to determine if further testing is needed to diagnose certain infections. This test may also be done to monitor the body's response to radiation therapy or chemotherapy.

How is the test performed?

In order to do a white blood cell count, a blood sample must be taken from a vein in the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein and the tourniquet is removed. Blood flowing from the vein through the needle is collected in a syringe or vial for tests in the lab. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

In the lab, a sample of blood is spread thinly on a slide and stained to highlight the blood cells. The number of white blood cells, or leukocytes, in the blood sample is counted. This allows the total number of leukocytes per microliter (mcl) of blood to be calculated.