The Test Sample
What is being tested?
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).
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
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.