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Understanding CD4 Cells and CD4 Cell Tests

July 2013

The Basics

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).

As the immune system loses CD4 cells, it becomes weaker and is less able to fight off germs. When it loses a large number of CD4 cells, people living with HIV (HIV+) become at risk of getting AIDS-related opportunistic infections (OIs), which are infections that 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 to 1,500 cells per cubic millimeter of blood (a cubic millimeter is a very small amount, about one drop). The number of CD4 cells a person has -- their "CD4 count" -- usually decreases as HIV disease progresses. Your CD4 cell count can help your health care provider tell whether your HIV disease is progressing, whether you should start HIV treatment, and whether the treatment you are taking is working well.

CD4 Count and HIV

Without HIV treatment, HIV infects and destroys more and more CD4 cells. As the CD4 count goes down, an HIV+ person becomes more likely to develop OIs and cancers.

CD4 Count and HIV Treatment

The US Department of Health and Human Services (DHHS) treatment guidelines now recommend treatment for all people living with HIV, no matter what their CD4 count.

The DHHS panel's recommendation to begin treatment with a CD4 count above 500 is a "moderately strong" recommendation based on expert opinion and not on data from clinical trials. The panel's recommendation to begin treatment with a CD4 count under 500 is based on the results of well-designed studies.

[Note: Because the panel's expert opinion is not based on results from randomized controlled trials, which are often considered the 'gold standard' for providing evidence, there is some debate about the DHHS panel's new recommendation to begin treatment with a CD4 count above 500. In other words, while some experts agree that the current level and quality of evidence supports treatment above 500, other experts do not.]

The DHHS treatment guidelines state:

Many people see their CD4 counts increase 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 may go up. However, your 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 drugs are working.

CD4 Cell Test

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.

It is important to get your CD4 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 count, including the time of day, level of 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. The second test would confirm any unexpected results or show that the first test's results were random -- the result of lab error or an unimportant occurrence. Try not to 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 to 60 percent.

The CD4 percentage is sometimes a more reliable measurement than the CD4 count because it tends to change less between measurements. While the CD4 percentage gives information about the health of your immune system, the CD4 count is the preferred measurement for assessing progression of HIV disease. Currently, treatment guidelines are based on CD4 counts and not on CD4 percentage.

If you begin treatment when your CD4 count and percentage are low (CD4 count <200), it may take longer for your CD4 numbers to increase. However, if your viral load is below detectable levels, then you can be encouraged that the virus is not growing or spreading. Sometimes watching your CD4 cells and percentage increase can take lots of patience and waiting time.

The Bottom Line

Because HIV infects and kills CD4 cells, CD4 counts usually drop as HIV disease gets worse. Taking effective combinations of HIV drugs may stop your CD4 count from dropping and your HIV disease from progressing.

Your CD4 count is an important indicator of the health of your immune system. Keeping track of the trends in your CD4 count can help you and your health care provider make decisions about starting and changing treatment. Getting regular CD4 cell tests -- along with viral load tests and other blood tests to check for treatment side effects -- is an important way to take charge of your health.

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