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 much damage has been done to your immune system and what's likely to happen next if you don't start taking antiretroviral treatment (ART). Everyone with HIV should take ART regardless of whether their CD4 count is high or low.
Doctors also track CD4 levels to check the effectiveness of your treatment and make adjustments as necessary. Your CD4 count should go up if your ART is effective.
Keeping your CD4 count up with ART can hold off symptoms and complications of HIV and help you live longer. Studies have found that people with HIV who stick to their treatments can live as long as people who do not have HIV.
What Does HIV Do to CD4 Cells?
HIV damages your immune system by targeting CD4 cells. The virus grabs on to the surface of a cell, gets inside, and becomes a part of it. When the infected CD4 cell dies, it releases more copies of HIV into your bloodstream.
Those new viruses 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 (OIs) because they take advantage of your body's lack of defense.
If you have very low CD4 counts, you may need to take drugs to prevent opportunistic infections in addition to taking ART. Once your CD4 count goes up in response to ART, you may be able to stop taking these OI medications.
How Do CD4 Levels Affect Viral Load?
The goal of ART is to lower the level of HIV in your blood – your “viral load” – to an undetectable level. This means the level of HIV in your blood is so low that it doesn’t show up in tests.
You still have the virus. And your HIV levels will grow again if you stop treatment. But properly followed, ART can control HIV levels so well that doctors can’t find HIV with typical lab tests.
The results are twofold.
First, as ART lowers your viral load, it gives your immune system a chance to make more CD4 cells. These help you fight infections and HIV-related cancers. Your doctor can tell because your CD4 levels (sometimes called T cell levels) start to go up.
Second, you become far less likely to pass HIV on to sexual partners. That’s because people who maintain an undetectable viral load with ART have almost no chance of passing HIV on to others through sex.
With careful ART treatment, many people can go on for decades or more without progressing to the third and most serious stage of HIV infection. That’s the stage known as acquired immunodeficiency syndrome, or AIDS.
ART is so effective that most people in the U.S. with HIV who get ART will never develop AIDS.
That’s why ART is so important. Without it, your HIV levels will grow and your CD4 levels will fall. Eventually, typically in about 10 years without treatment, you will develop AIDS.
This is very serious. People with AIDS who don’t get treatment survive about 3 years. And with AIDS, you’re also more likely to have a high viral load that spreads more easily to sexual partners.
What the Results Mean
A normal CD4 count is from 500 to 1,400 cells per cubic millimeter of blood. CD4 counts go down over time if you do not take ART.
At levels below 200 cells per cubic millimeter, you are more likely to get to a wide variety of OIs, many of which can be deadly.
Your CD4 levels behave differently depending on your stage of HIV:
Stage 1: Acute HIV Infection. HIV is reproducing in large amounts and destroying CD4 cells. That’s why CD4 levels typically fall quickly at first. Then, as your immune system responds, your viral load begins to fall and your CD4 levels start to rise again. But they might not return to pre-infection levels.
Stage 2: Chronic HIV Infection. HIV is still active but reproduces much more slowly. ART treatment can keep you in this stage for many decades. This can help maintain CD4 at healthy levels, sometimes indefinitely. If this period starts to head toward stage 3, your viral load goes up and CD4 levels go down.
Stage 3: AIDS. Your immune system is damaged badly enough to allow opportunistic infections of different types. Your doctor might diagnose AIDS because of these. But your doctor can also diagnose this stage by CD4 levels. Once they get below 200, most doctors will diagnose AIDS.
No matter what your stage, test results don't always match how well you're feeling. Some people can have high CD4 counts and be doing poorly for other reasons. Others can have low CD4 counts but feel well, with few complications. But those patients are at risk of becoming very ill if they don’t start HIV treatment.
Anyone who is HIV-positive should take antiretroviral therapy medications regardless of their CD4 count and 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 despite taking ART, your virus might be developing resistance to the drugs that you are taking. This should be apparent from HIV viral load tests, which your doctor should be doing every few months. In this case, your doctor may want to change your ART drugs.
What Else Can Affect Your CD4 Count
Things other than the HIV virus can influence how high or low your CD4 count is.
Even things as simple as smoking or changes in sleep or exercise habits can make a difference in the count. Regular alcohol use can also lower CD4 counts.
To get the most accurate and helpful results for your CD4 count, try to:
- Get tested on a normal day after typical sleep and exercise.
- Use the same lab each time.
- 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.
It may also show the need for drugs to prevent specific OIs in addition to the drugs you are taking for HIV.
A viral load test 2 to 8 weeks after you start or change treatment helps your doctor decide how well the ART is working. A CD4 test will show whether the immune system is improving in response to ART.
Then you should typically get a CD4 test every 3 to 6 months, or as often as your doctor recommends, to see how well your immune system is doing. If your CD4 counts are low and you take drugs for specific OIs in addition to your ART, you may be able to stop the OI drugs as your immune system responds to ART.
If your CD4 counts stay above 500 and you maintain viral suppression, you may not need further CD4 testing unless there is a change in your health.