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, and is usually ordered as part of a complete blood count (CBC).
This test is normally performed to detect an infection or inflammation. A healthcare professional may also use this test to determine if further testing is needed to diagnose certain infections. This test is also essential for monitoring the body's response to radiation therapy or chemotherapy.
A blood sample must be taken from a vein, usually in the arm. 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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 placed in a special machine that counts the white blood cells. This allows the total number of WBCs per microliter (mcl) of blood to be calculated.
A person should request specific instructions from his or her healthcare professional. Normally, no preparation is required.
A normal white blood cell count ranges from 4,500 to 10,000 cells/mcl (cells per microliter) of blood. Abnormally low numbers of white blood cells do not always represent a disease process, but may be caused by: bone marrow failurea bone marrow tumora substance in the body that is toxic to these cellscollagen-vascular diseases, such as rheumatoid arthritis or systemic lupus erythematosus liver diseasespleen diseaseradiation
Abnormally high numbers of white blood cells may be caused by: allergyemotional or physical stressinfectious diseasesinflammatory diseasesleukemiatissue damage
The differential WBC count can further clarify the causes, if they are not obvious already from the history and physical exam.