At a Glance
Why Get Tested?
When to Get Tested?
If you’ve been diagnosed with HIV, soon after you are first diagnosed to get a baseline assessment of your immune system; 2-8 weeks after starting anti-HIV therapy and then every three to four months if you continue therapy
A blood sample taken from a vein in your arm
Test Preparation Needed?
No test preparation is needed.
The Test Sample
What is being tested?
CD4 and CD8 cells are lymphocytes that have markers on the surfaces of the cells called CD4 and CD8. They are types of white blood cells that fight infection, and they play an important role in your immune system function. CD4 and CD8 cells are made in the bone marrow, and are educated in the thymus gland, a small gland found in the upper chest. They circulate throughout the body in the bloodstream, spleen and the lymph nodes.
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 the HIV virus 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. As treatment reduces the amount of HIV present in the body and slows progression, the CD4 count will increase and/or stabilize.
CD8 cells include lymphocytes that are called cytotoxic T cells and T-suppressor cells [S12]. CD8 cytotoxic T-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.
These tests measure the number of CD4 and CD8 cells in the blood and, in conjunction with an HIV viral load test, assess the status of the immune system if you have been diagnosed with HIV. As the 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.
These 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 FAQ #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.
Ask a Laboratory Scientist
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.