A WBC count is a blood test to measure the number of white blood cells (WBCs) in the blood.
WBCs are also called leukocytes. They help fight infections. There are 5 major types of white blood cells:
- Lymphocytes (T cells, B cells, and Natural Killer cells)
How the Test is Performed
A blood sample is needed.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.
How to Prepare for the Test
Most of the time, you do not need to take special steps before this test. Tell your health care provider the medicines you are taking, including the ones without a prescription. Some drugs may change the test results.
The normal number of WBCs in the blood is 4,500 to 11,000 WBC per microliter (4.5 to 11.0 × 109/L).
Normal value ranges may vary slightly among different labs. Some labs use different measurements or may test different specimens. Talk to your provider about your test results.
Veins and arteries vary in size from one person 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.
Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
What Abnormal Results Mean
LOW WBC COUNT
A low number of WBCs is called leukopenia. A count less than 4,500 cells per microliter (4.5 × 109/L) is below normal.
Neutrophils are one type of WBC. They are important for fighting infections.
A lower than normal WBC count may be due to:
- Bone marrow deficiency or failure (for example, due to infection, tumor, or abnormal scarring)
- Cancer treating drugs, or other medicines (see list below)
- Certain autoimmune disorders such as lupus (SLE)
- Disease of the liver or spleen
- Radiation treatment for cancer
- Certain viral illnesses, such as mononucleosis (mono)
- Cancers that damage the bone marrow
- Very severe bacterial infections
- Severe emotional or physical stress (such as from an injury or surgery)
HIGH WBC COUNT
A higher than normal WBC count is called leukocytosis. It may be due to:
- Certain drugs or medicines (see list below)
- Cigarette smoking
- After spleen removal surgery
- Infections, most often those caused by bacteria
- Inflammatory disease (such as rheumatoid arthritis or allergy)
- Leukemia or Hodgkin disease
- Tissue damage (for example, burns)
There may also be less common reasons for abnormal WBC counts.
Drugs that may lower your WBC count include:
- Antithyroid drugs
- Chemotherapy drugs
- Diuretics (water pills)
- Histamine-2 blockers
Drugs that may increase WBC counts include:
- Beta adrenergic agonists (for example, albuterol)
- Granulocyte colony stimulating factor
Why the Test is Performed
You will have this test to find out how many WBCs you have. Your provider may order this test to help diagnose conditions such as:
- An infection
- Allergic reaction
- Blood cancer such as leukemia or lymphoma
- Side effects caused by medicines
Chernecky CC, Berger BJ. Differential leukocyte count (Diff) - peripheral blood. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:441-450.
Vajpayee N, Graham SS, Bem S. Basic examination of blood and bone marrow. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 30.