Your immune system protects your body by fighting germs and infections. White blood cells are an important part of your immune system. HIV infects and destroys a type of white blood cell called a CD4 cell (sometimes called a T-cell).

When the immune system loses too many CD4 cells, it becomes weak and is unable to fight off germs. At this point, you are at risk of getting infections, called AIDS-related opportunistic infections (OIs), which can cause serious illness or death.

The number of CD4 cells you have gives a picture of the health of your immune system. A normal CD4 cell count is about 500-1,500 cells per cubic millimeter of blood (a cubic millimeter is a very small amount, about one drop). CD4 cell counts usually fall as HIV disease progresses. Your CD4 cell count can help your health care provider tell whether your HIV disease is progressing and whether you should start HIV treatment.

Without HIV medication, HIV infects more and more cells. This causes the CD4 count to go down, usually by about 50 to 100 cells per year, in most HIV+ people. As the CD4 count goes down, an HIV+ person becomes more likely to develop OIs and cancers.

• Above 500: People with CD4 counts above 500 cells usually have a fairly normal immune system and are at low risk for OIs
• Below 200: People with CD4 counts below 200 cells are diagnosed as having AIDS and are at risk for developing serious OIs such as Pneumocystis pneumonia (PCP), Mycobacterium avium complex (MAC), and cytomegalovirus (CMV)

People with low CD4 counts are at risk for developing serious OIs such as Pneumocystis pneumonia (PCP), Mycobacterium avium complex (MAC), and cytomegalovirus (CMV). As CD4 cells drop below 200, your health care provider will recommend medications to prevent these infections.

The US government guidelines recommend that people start HIV treatment before their CD4 cell counts fall below 350 (and some experts recommend starting HIV treatment at 500 CD4 cells). Many people have CD4 cell increases when they start effective HIV treatment. If the drugs succeed in slowing or stopping HIV, fewer new CD4 cells will be infected and your CD4 count can go up. But the CD4 count can also go down again if you stop taking your HIV drugs correctly, or if your HIV becomes resistant to the drugs. Along with your viral load, your CD4 count is a very valuable tool for monitoring your HIV disease and how well your HIV medicines are working.

The US government guidelines state:

• HIV treatment should be started in anyone with a CD4 count less than 350
• HIV treatment should also be started if you are in one or more of the following situations, no matter what your CD4 count:
  • You have or had symptoms of AIDS (such as OIs)
  • You are a pregnant woman
  • You have HIV-related kidney disease
  • You need treatment for hepatitis B (HBV)
• HIV treatment is recommended to be started in anyone with a CD4 cell count between 350 and 500
• Some experts believe that treatment should be start even earlier – with a CD4 count above 500 and no symptoms

Many people have CD4 cell increases when they start effective HIV treatment. If the drugs succeed in slowing or stopping HIV, fewer new CD4 cells will be infected and your CD4 count can go up. But the CD4 count can also go down again if you stop taking your HIV drugs correctly, or if your HIV becomes resistant to the drugs. If your CD4 cells drop below 200, your health care provider will recommend medications to prevent OIs.

Your CD4 count is a very valuable tool for monitoring your HIV disease. It is used along with viral load testing to check how well your HIV medicines are working and to determine if you need to make changes to your HIV treatment regimen.

The CD4 cell test is a simple blood test ordered by your health care provider. When you first start receiving care for HIV, you should get a “baseline” CD4 cell test. This baseline test gives a picture of your immune system when you first enter into care. Later tests can be compared against this first result to see how things are changing over time and with treatment.

You should get your CD4 cell count checked about every three to six months--or as often as your health care provider recommends. You will need more frequent CD4 cell tests if your count is low or falling, or if you are starting or changing treatment.

Many factors can affect your CD4 cell count, including the time of day, stress, your menstrual cycle, and infections such as the flu. If you get a result back that surprises you or your health care provider, he or she will probably want you to get a second test. That second test would confirm any unexpected results or prove that those results were random, sort of a fluke (that is, not important). Don’t worry too much about a single abnormal test result; trends over time are usually more important.

In addition to your CD4 cell count, your health care provider will want to check your CD4 percentage. This number tells you what percent of your total white blood cells are CD4 cells. A normal CD4 cell percentage is about 30-60 percent.

The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to change less between measurements. There are no guidelines for treatment decisions based on CD4 percentage. However, a CD4 percentage below 14 percent is a definition of AIDS. It usually means that the immune system is damaged and it is time to begin treatment to prevent OIs like PCP.

Because HIV attacks CD4 cells, CD4 counts usually drop as HIV disease gets worse. Taking effective combinations of HIV drugs may stop CD4 cell count decreases and HIV disease progression.

Your CD4 cell count is an important indicator of the health of your immune system. Tracking trends in your CD4 cell count can help you make decisions about starting and switching treatment. Getting regular CD4 cell tests—along with viral load tests and other blood tests to monitor treatment side effects—is an important way to take charge of your health.