March 21, 2009
Researchers can tell these cells apart by specific proteins on the cell surface. A T-4 cell is a T-cell with CD4 molecules on its surface. This type of T-cell is also called "CD4 positive," or CD4.
When someone is infected with HIV for a long time, the number of CD4 cells they have (their CD4 cell count) goes down. This is a sign that the immune system is being weakened. The lower the CD4 cell count, the more likely the person will get sick.
There are millions of different families of CD4 cells. Each family is designed to fight a specific type of germ. When HIV reduces the number of CD4 cells, some of these families can be totally wiped out. You can lose the ability to fight off the particular germs those families were designed for. If this happens, you might develop an opportunistic infection (see Fact Sheet 500).
Infections can have a large impact on CD4 cell counts. When your body fights an infection, the number of white blood cells (lymphocytes) goes up. CD4 counts go up, too. Vaccinations can cause the same effects. It's best to wait a couple of weeks after you recover from an infection or get a vaccination before you get a CD4 test.
Because the CD4 cell counts are so variable, some health care providers prefer to look at the CD4 cell percentages. These percentages refer to total lymphocytes. If your test reports CD4% = 34%, that means that 34% of your lymphocytes were CD4 cells. This percentage is more stable than the number of CD4 cells. The normal range is somewhere between 30% and 60%. Different labs use different ranges. There are no guidelines for treatment decisions based on CD4%. However, a CD4% below 14% is a definition of AIDS.
A CD4 count below 200 indicates serious immune damage. It is a sign of AIDS in people with HIV infection. A recent study showed that the CD4% is a better predictor of HIV disease progression than the CD4 count. However, the CD4 count is used to decide when to start treatment.
CD4 counts are used together with the viral load to estimate how long someone will stay healthy. See Fact Sheet 125 for more information on the viral load test.
CD4 counts are also used to indicate when to start certain types of drug therapy:
When to start antiretroviral therapy (ART): When the CD4 count goes below 350, most health care providers begin ART (see Fact Sheet 403). Some health care providers use the CD4% going below 15% as a sign to start aggressive antiviral therapy, even if the CD4 count is high. More conservative doctors might wait until the CD4 count drops to near 200 before starting treatment. A recent study found that starting treatment with a CD4% below 5% was strongly linked to a poor outcome.
When to start drugs to prevent opportunistic infections: Most health care providers prescribe drugs to prevent opportunistic infections at the following CD4 levels:
Monitoring treatment success: With successful ART, CD4 counts rise. Sometimes they rise quickly. Other times they can go up slowly. If the CD4 count is below when ART is started, CD4 counts might not recover to normal levels. Also, if CD4 counts fall while you're taking ART, you might have to change your medications.
Now that people with AIDS are living are longer, there is more research on other causes of illness and death. These "non-AIDS" causes of death include liver disease, non-AIDS cancers, and heart disease. Overall, these deaths are decreasing. However, research shows a clear link between lower CD4 counts and the risk of death.
Because they are such an important indicator of the strength of the immune system, official treatment guidelines in the US suggest that CD4 counts be monitored every 3 to 4 months. See Fact Sheet 404 for more information on the treatment guidelines.