White Blood Cell Count (WBC)
As part of a complete blood count (CBC), when you have a routine health examination; when you have any number of signs and symptoms that may be related to a condition affecting WBCs such as infection, inflammation, or cancer; when you have a condition or are receiving treatment that is known to affect WBCs
A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)
White blood cells, also called leukocytes, are cells that exist in the blood, the lymphatic system, and tissues and are an important part of the body's defense system. They help protect against infections and also have a role in inflammation, allergic responses, and protecting against cancer. The white blood cell (WBC) count totals the number of white blood cells in a person's sample of blood. It is one test among several that is included in a complete blood count (CBC), which is often used in the general evaluation of a person's health.
Blood is made up of a few different types of cells suspended in fluid called plasma. In addition to WBCs, there are red blood cells and platelets. All of these cells are produced in the bone marrow and subsequently released into the blood to circulate. There are five types of WBCs, and each has a different function.
Three types of WBCs are referred to as "granulocytes" because of the granules present in their cytoplasm. These granules release chemicals and other substances as part of the immune response. Granulocytes include neutrophils, which normally make up the largest number of circulating WBCs, eosinophils, and basophils. The other two types of WBCs are monocytes and lymphocytes. Lymphocytes are further divided into three subtypes: B lymphocytes that produce antibodies (also known as immunoglobulins), T lymphocytes, and natural killer cells (NK cells).
When there is an infection or an inflammatory process somewhere in the body, the bone marrow produces more WBCs, releasing them into the blood, and through a complex process, they move to the site of infection or inflammation. As the condition resolves, the production of WBCs by the bone marrow subsides and the number of WBCs drops to normal levels again.
In addition to infections and inflammation, there are a number of conditions that can affect the production of WBCs by the bone marrow or the survival of WBCs in the blood, such as cancer or an immune disorder, resulting in either increased or decreased numbers of WBCs in the blood. The WBC count, along with the other components of the CBC, alerts a health practitioner to possible health issues. Results are often interpreted in conjunction with additional tests such as a WBC differential and a blood smear review. A differential may inform the health practitioner as to which type of WBC may be low or high, and a blood smear can reveal the presence of abnormal and/or immature populations of WBCs.
If results indicate a problem, a wide variety of other tests maybe performed in order to help determine the cause. A health practitioner will typically consider an individual's signs and symptoms, medical history, and results of a physical examination to decide what other tests may be necessary. For example, as needed, a bone marrow biopsy will be performed to evaluate the bone marrow status.
How is the sample collected for testing?
A blood sample is obtained by a needle placed in a vein in the arm or a fingerstick (for children and adults) or heelstick (for newborns).
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed. Tell your healthcare provider about any medications (prescription and over-the-counter) that you have taken recently. Some medications can cause an increase or decrease in WBCs.
How is the test used?
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.
When is it ordered?
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
- 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.
What does the test result mean?
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.
Is there anything else I should know?
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.
Can a WBC count be performed on a sample other than blood?
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.
If I have an abnormal WBC count, what other tests might my doctor order?
Other general tests to check your health may include a comprehensive metabolic panel (CMP). Depending on your signs, symptoms, medical history, physical exam and suspected condition, your healthcare provider may choose to order a variety of other tests. A few general examples include:
- Bacterial infection: a culture of the affected area (e.g., urine culture, sputum culture, blood culture), strep test
- Viral infection: tests for mononucleosis, EBV
- Inflammation: CRP, ESR
- Autoimmune diseases: ANA
- Allergies: Allergy tests
- Leukemia: Bone marrow biopsy, immunophenotyping (e.g., flow cytometry), chromosome analysis
On This Site
Tests: Complete Blood Count, Blood Smear, White Blood Cell Differential Count, Body Fluid Analysis, Bone Marrow Aspiration and Biopsy, Immunophenotyping, Chromosome Analysis
Conditions: HIV, Leukemia, Meningitis and Encephalitis, Multiple Myeloma, Myeloproliferative Neoplasms, Allergies, Asthma, Autoimmune Disorders, Inflammatory Bowel Disease, Rheumatoid Arthritis
Elsewhere On The Web
American Society of Hematology: Blood Disorders
KidsHealth.org: Immune System
National Institute of Allergy and Infectious Diseases: Immune System
MedlinePlus Medical Encyclopedia: Immune Response
Mayo Clinic: High White Cell Count
Mayo Clinic: Low White Cell Count
National Cancer Institute: Leukemia