March 29, 2012
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 but has not started treatment, the number of CD4 cells they have 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 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).
Current treatment guidelines (see Fact Sheet 404) suggest monitoring CD4 tests every 3 to 6 months when starting antiretroviral therapy (ART, see Fact Sheet 403). Once treatment has increased CD4 levels to high levels, the test should be done every 6 to 12 months.
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. 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. Although the CD4% may be a better predictor of HIV disease progression than the CD4 count, the CD4 count is used to decide when to start treatment.
CD4 counts are also used to indicate when to start certain types of drug therapy:
When to start drugs to prevent opportunistic infections:
Most health care providers prescribe drugs to prevent opportunistic infections at the following CD4 levels:
Less than 200: pneumocystis pneumonia (PCP; see Fact Sheet 515)
Less than 100: toxoplasmosis (see Fact Sheet 517) and cryptococcosis (see Fact Sheet 503)
Less than 50: mycobacterium avium complex (MAC; see Fact Sheet 514)
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 low 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.
Higher CD4 counts are better. However, a normal CD4 count does not guarantee a normal immune system.
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 6 to 12 months for people whose ART is working to maintain a high level of CD4 cells.