The Test Sample
What is being tested?
CD4 and CD8 cells are types of white blood cells that fight infection and play an important role in immune system function. They are made in the spleen, lymph nodes, and thymus gland, and they circulate throughout the body in the bloodstream. CD4 and CD8 cells have markers on the surfaces of the cells called clusters of differentiation (CD), and the number identifies the specific type of cell. CD4 and CD8 tests measure the number of these cells in the blood and, in conjunction with an HIV viral load test, help assess the status of the immune system in a person who has been diagnosed with human immunodeficiency virus (HIV).
CD4 cells are sometimes called T-helper cells. They help to identify, attack, and destroy specific bacteria, fungi, and viruses that affect the body. CD4 cells are a major target for HIV, which binds to the surface of CD4 cells, enters them, and either replicates immediately, killing the cells in the process, or remains in a resting state, replicating later. As HIV gets into the cells and replicates, the number of CD4 cells in the blood gradually declines. The CD4 count decreases with HIV disease progression. This process may continue for several years before the number of CD4 cells drops to a low enough level that symptoms associated with AIDS begin to appear. AIDS treatment reduces the amount of HIV present in the body and slows disease progression. When this occurs, the CD4 count will increase and/or stabilize.
CD8 cells are lymphocytes that are sometimes called T-suppressor cells or cytotoxic T cells. CD8 cells identify and kill cells that have been infected with viruses or that have been affected by cancer. They play an important role in the immune response to HIV by killing cells infected with the virus and by producing substances that block HIV replication.
As HIV disease progresses, the number of CD4 cells will decrease in relation to the number of total lymphocytes and CD8 cells. To provide a clearer picture of the condition of the immune system, the results of these tests may be expressed as a ratio of CD4 to total lymphocytes (percentage) or as a ratio of CD4 cells to CD8 cells.
CD4 and CD8 tests are most often used to help monitor disease progression in HIV but may also be used occasionally in other conditions, such as lymphomas and organ transplantation. (See Common Questions #4)
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
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.