CD4 and CD8

If you have been diagnosed with HIV, a CD4 count 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 the CD4 cell count may be compared to the CD8 cell count, and the result is expressed as a ratio. However the absolute CD4 T-cell count is the most important value.

The CD4 count and 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 CD4 counts.  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, DiGeorge syndrome, and in organ transplantation. (See FAQ #4).

A CD4 count, or sometimes a 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.  However the absolute CD4 T-cell count is the most important value.

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 WHO 2006 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 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.

A 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 counts have few medical complications and function well.