The CD4 count is a test that measures how many CD4 cells you have in your blood. These are a type of white blood cell, called T-cells, that move throughout your body to find and destroy bacteria, viruses, and other invading germs.
Your test results help your doctor know how strong your immune system is and guide HIV treatment choices. Your CD4 count can also be used to tell what stage of HIV or AIDS you have and what's likely to happen next.
What Does HIV Do to CD4 Cells?
HIV damages your immune system because it targets CD4 cells. The virus grabs on to the surface of a cell, gets inside, and becomes a part of it. As an infected CD4 cell multiplies so it can do its job, it also makes more copies of HIV.
Those new bits of virus find and take over more CD4 cells, and the cycle continues. This leads to fewer and fewer HIV-free, working CD4 cells.
HIV can destroy entire "families" of CD4 cells, and then the germs these cells fight have easy access to your body. The resulting illnesses are called opportunistic infections because they take advantage of your body's lack of defense.
What the Results Mean
A normal CD4 count is from 500 to 1,400 cells per cubic millimeter of blood. For some people, CD4 counts can drop dramatically, even going down to zero.
But it's more important to pay attention to the pattern of your results than to any one test. In general, your HIV infection is getting worse if your CD4 count is going down. It means your immune system is getting weaker and you're more likely to get sick.
The test results don't always match how well you're feeling though. Some people can have high CD4 counts and be doing poorly. Others can have low CD4 counts but few complications.
Anyone who is HIV-positive should take antiretroviral therapy (ART) medications, whether or not they have symptoms. When your treatment is working, your CD4 count should stay steady or go up.
If your CD4 count keeps going down over several months, your doctor may want to change your ART drugs or start treatment to prevent opportunistic infections.
What Else Can Affect Your CD4 Count
Things other than the HIV virus can influence how high or low your CD4 count is, too.
It tends to be lower in the morning and higher in the evening. Fatigue and stress can skew test results.
An infection like the flu, pneumonia, or a herpes simplex virus (including cold sores) can make your CD4 count go down for a while. Getting a vaccination can also change it while your body learns how to fight that sickness.
To get the most accurate and helpful results for your CD4 count, try to:
- Use the same lab each time.
- Have your tests done at the same time of day.
- Wait for at least a couple of weeks after you've been sick or gotten a shot before you get a test.
When to Get a Test
Right after you're diagnosed, you should get a CD4 count for a "baseline measurement." That gives your doctor something to compare future test results to.
A test 2 to 8 weeks after you start or change treatment helps your doctor decide how well the medication is working.
Then you should typically get a test every 3 to 6 months, or as often as your doctor recommends, to see how well your immune system is doing.