White Blood Cell Count (WBC)
As part of a complete blood count (CBC), when you have a routine health examination; when you have signs and symptoms that may be related to a condition affecting the number of WBCs; when you have a condition or are receiving treatment that is known to affect WBCs
A blood sample drawn from a vein or by a fingerstick (children and adults) or heelstick (newborns)
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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, and allergic reactions. The white blood cell (WBC) count totals the number of white blood cells in a sample of your 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 your health.
Blood is made up of three main types of cells suspended in fluid called plasma. In addition to WBCs, there are red blood cells and platelets. All of these cells are made in the bone marrow and are 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 (neu) normally make up the largest number of circulating WBCs. They move into an area of damaged or infected tissue, where they engulf and destroy bacteria or sometimes fungi.
- Eosinophils (eos) respond to infections caused by parasites, play a role in allergic reactions (hypersensitivities), and control the extent of immune responses and inflammation.
- Basophils (baso) usually make up the fewest number of circulating WBCs and are thought to be involved in allergic reactions.
- Lymphocytes (lymphs) exist in both the blood and the lymphatic system. They are divided into three types:
- B lymphocytes (B cells) produce antibodies as part of the body’s natural defense (immune) responses.
- T lymphocytes (T cells) recognize foreign substances and process them for removal.
- Natural killer cells (NK cells) directly attack and kill abnormal cells such as cancer cells or those infected with a virus.
- Monocytes (mono), similar to neutrophils, move to an area of infection and engulf and destroy bacteria. They are associated more often with chronic rather than acute infections.
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 healthcare 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 provide information on which type of WBC may be low or high, and a blood smear may show the presence of abnormal and/or immature WBCs.
If results indicate a problem, a wide variety of other tests can be performed to help determine the cause. A healthcare 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 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 or other diseases that affect the number of 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.
A WBC count can be used to detect is a disease or condition affecting white blood cells, but it cannot determine the underlying cause. Several other tests may be done help make a diagnosis, such as 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 you have a routine health examination. The test may be done when you have 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 healthcare practitioner thinks may be related to a blood disorder, autoimmune disorder, or an immune deficiency.
A WBC may be ordered on a regular basis when you have been diagnosed with an infection, blood or immune disorder or another condition affecting the number and types of WBCs. It may also be ordered periodically when you are receiving treatment for one of these conditions or when you are receiving 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 healthcare 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 (e.g., vitamin B12 deficiency)
- Overwhelming infections (e.g., sepsis)
- Diseases of the immune system, such as HIV, which destroy T lymphocytes
When WBC counts are used for monitoring medical conditions, 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.
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 your healthcare practitioner can use 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, Epstein-Barr virus (EBV)
- Inflammation: CRP (preferred), ESR
- Autoimmune diseases: ANA
- Allergies: Allergy tests
- Leukemia: Bone marrow biopsy, immunophenotyping (e.g., flow cytometry), chromosome analysis
Is there anything else I should know?
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 types of medications (prescription and over the counter) that cause both increased and decreased WBC counts.