If you have been diagnosed with HIV, a CD4 test by itself, a CD4 percent, or a CD4/CD8 ratio is used to help evaluate and track the progression of HIV infection and disease. CD4 cells are the main target of HIV, and the number of CD4 cells will decrease as HIV progresses. Since CD4 cells are usually destroyed more rapidly than other types of lymphocytes and because absolute counts can vary from day to day, it is sometimes useful to look at the number of CD4 cells compared to other types of lymphocytes. Sometimes a CD4 cell count is compared to the total lymphocyte count, and the result is expressed as a percentage, or a CD4 cell count may be compared to a CD8 cell count, and the result is expressed as a ratio.

The CD4 count, CD4 percent, or a CD4/CD8 ratio can tell your doctor how strong your immune system is and can help predict the risk of complications and debilitating infections. These tests are most useful when they are compared with results obtained from earlier tests. They are used in combination with the HIV viral load test, which measures the level of HIV in the blood, to determine the progression and outlook of HIV disease and to monitor the effectiveness of treatment.

Sometimes, these tests may be used to help diagnose or monitor other conditions such as lymphoma, organ transplantation, and DiGeorge syndrome (see FAQ #4).

A CD4 count, or sometimes a CD4 percent or CD4/CD8 ratio, is ordered with a viral load test when you are first diagnosed with HIV as part of a baseline measurement. The tests should be repeated about two to eight weeks after starting or changing anti-HIV therapy. If treatment is maintained, they should be performed about every three to four months thereafter.

A CD4 count may be interpreted as an absolute level, as a ratio to CD8, or as a percent of total lymphocytes. In this section, we use CD4 count to refer to any of these ways of expressing it. In general, the CD4 count goes down as HIV disease progresses. Any single CD4 count value may differ from the last one even though your health status has not changed. Your doctor will take several CD4 test results into account rather than a single value and will evaluate the pattern of CD4 counts over time.

If your CD4 count declines over several months, your doctor may recommend beginning or changing anti-retroviral treatment and/or starting prophylactic treatment for opportunistic infections such as Pneumocystis pneumonia (PCP) or Mycobacterum avium infection. Your CD4 count should increase or stabilize in response to effective combination anti-HIV therapy.

According to public health guidelines, preventive therapy should be started when an HIV-positive person who has no symptoms registers a CD4 count under 200 cells per cubic millimeter of blood. Some physicians will opt to consider treatment earlier, at 350 cells/mm³. The Centers for Disease Control and Prevention considers HIV-infected persons who have CD4 counts below 200 cells/mm³ to have AIDS, regardless of whether they have any signs or symptoms.

The CD4 count tends to be lower in the morning and higher in the evening. Acute illnesses, such as pneumonia, influenza, or herpes simplex virus infection, can cause the CD4 count to decline temporarily. Cancer chemotherapy can dramatically lower the CD4 count.

The CD4 count does not always reflect how someone with HIV disease feels and functions. For example, some people with higher counts are ill and have frequent complications, and some people with lower CD4 counts have few medical complications and function well.