White Blood Cell Count (WBC)

The white blood cell count (WBC) is used as part of a full complete blood count (CBC) to:

• Screen for a wide range of diseases and conditions
• Help diagnose an infection or inflammatory process; it also may be used to determine the presence of other diseases that affect WBCs such as allergies, leukemia or immune disorders, to name a few.
• Monitor the progression of conditions such as those named above; monitor the body's response to various treatments and/or to monitor bone marrow function; some treatments, such as radiation and chemotherapy, are known to affect white cells and may be monitored using WBC counts.

White blood cells exist in the blood, the lymphatic system, and tissues and are an important part of the body's defense system. (See the "What is being tested?" section for more.) Some diseases trigger a response by the immune system and cause an increase in the number of WBCs. Other conditions affect the production of WBCs by the bone marrow or the survival of WBCs in the circulation, resulting in either an increase or decrease in the number of circulating WBCs.

A WBC count can indicate that there is a disease or condition affecting white blood cells, but it cannot determine the underlying cause. Several other tests may be performed at the same time or in follow up to an abnormal result to help make a diagnosis. Some of these additional tests may include a WBC differential, a blood smear review, or in severe conditions, a bone marrow examination. A differential may indicate which type of WBC is low or high while a blood smear and/or bone marrow biopsy can reveal the presence of abnormal and/or immature WBCs.

A WBC count is normally ordered as part of the complete blood count (CBC), which may be performed when an individual undergoes a routine health examination. The test may be done when someone has general signs and symptoms of an infection and/or inflammation such as:

• Fever, chills
• Body aches, pain
• Headache
• A variety of other signs and symptoms, depending on the site of suspected infection or inflammation

Testing may be performed when there are signs and symptoms that a health practitioner thinks may be related to a blood disorder, autoimmune disorder, or an immune deficiency.

A WBC may be ordered often and on a regular basis to monitor an individual who has been diagnosed with an infection, blood or immune disorder or another condition affecting WBCs. It may also been ordered periodically to monitor the effectiveness of treatment or when a particular therapy is known to affect WBCs, such as radiation or chemotherapy.

A WBC count indicates an overall increase or decrease in the number of white blood cells. A health practitioner will consider the results of a WBC count together with results from other components of the complete blood count (CBC) as well as a number of other factors, such as physical examination, medical history, and signs and symptoms.

A high white blood cell count, called leukocytosis, may result from a number of conditions and diseases. Some examples include:

• Infections, most commonly caused by bacteria and some viruses, less commonly by fungi or parasites
• Inflammation or inflammatory conditions such as rheumatoid arthritis, vasculitis or inflammatory bowel disease
• Leukemia, myeloproliferative neoplasms
• Conditions that result in tissue death (necrosis) such as trauma, burns, surgery or heart attack
• Allergic responses (e.g., allergies, asthma)

A low white blood cell count, called leukopenia, can result from conditions such as:

• Bone marrow damage (e.g., toxin, chemotherapy, radiation therapy, drugs)
• Bone marrow disorders—the bone marrow does not produce sufficient WBCs (e.g., myelodysplastic syndrome, vitamin B12 or folate deficiency)
• Lymphoma or other cancer that has spread (metastasized) to the bone marrow
• Autoimmune disorders—the body attacks and destroys its own WBCs (e.g., lupus)
• Dietary deficiencies
• Overwhelming infections (e.g., sepsis)
• Diseases of the immune system, such as HIV, which destroy T lymphocytes

When WBC counts are used for monitoring purposes, a series of WBC counts that continues to rise or fall to abnormal levels indicates that the condition or disease is getting worse. WBC counts that return to normal indicate improvement and/or successful treatment.

If you have had your spleen removed, you may have a persistent mild to moderate increase in WBC count.

Intense exercise or severe emotional or physical stress can increase a WBC count, but the test is not used to evaluate these conditions. Pregnancy in the final month and labor may also be associated with increased WBC levels.

In the U.S. population, WBC counts are related to one's age, sex, ethnicity, and smoking status. It is not uncommon for the elderly to fail to develop high WBC count (leukocytosis) as a response to infection.

There are many drugs that cause both increased and decreased WBC counts.

Yes. WBC counts may be performed on many different types of body fluids. A common reason that this is done is to more directly assess one area of the body that may be infected or inflamed. For example, if meningitis is suspected, then a WBC count plus differential may be performed on a sample of cerebrospinal fluid (CSF). Many other examples are listed in the article on Body Fluid Analysis.