CD4 and CD8

Other names: CD4 Lymphocyte Count; CD4 Percent; T-Cell Subsets; CD8 Lymphocyte Count; CD4/CD8 Ratio; T4 Count; T-Suppressor Cells; T-Helper Cells; Cytotoxic T-Cells

CD4 and CD8 are types of white blood cells, known as lymphocytes, which have markers on the surface known as CD4 and CD8. CD4 cells are commonly known as T-helper cells; they help to detect and fight off bacterial and viral infections. The CD4 cells are targeted by the HIV virus; as the virus reproduces it causes the number of CD4 cells to decrease. CD8 cells detect and try to fight off infections caused by viruses or disease such as cancer. CD8 cells are important for making the body resistant to HIV because they help to encourage the body to stop the virus replicating.

When is the test used?

The CD4 and CD8 test is primarily used to assess the condition of a patient who has already been diagnosed with HIV. The test is ordered to track the progress of the virus and monitor the body’s response. The test measures the number of CD4 cells, which gives doctors an idea of how rapidly the virus is replicating. Doctors may repeat the test several times. The CD4 count and the CD4:CD8 ratio helps to tell doctors how strong the patient’s immune system is.

In some cases, the test may also be used to diagnose specific conditions, including lymphoma and DiGeorge syndrome.

How is the test performed?

The test is done by collecting and analysing a sample of blood taken from a vein in the arm. The blood is collected using a needle, which is inserted into a vein in the arm; the blood is then collected in a syringe and placed in a sample bottle; it can then be labelled and sent away to the laboratory for analysis.

What do the test results mean?

Normally, the CD4 count decreases as the HIV becomes more advanced. If the CD4 count decreases over a period of a few months, the patient will usually begin anti-retroviral treatment or prophylactic treatment and then the test will be repeated to monitor the progress of the treatment; as the treatment starts to work the CD4 count should stabilise.

The CD4 count may also be decreased as a result of chemotherapy treatment for cancer; levels may also vary according to the time of day (levels are usually higher in the evening). Levels of CD4 may also be affected by other illnesses including influenza and pneumonia.

Specific Blood Tests

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