CD4 count: What does it mean?
CD4 cells are a type of white blood cell that fight infection. Another name for them is T-helper cells. CD4 cells are made in the spleen, lymph nodes and the thymus gland, which are part of the lymph, or infection-fighting, system. CD4 cells move throughout your body, helping to identify and destroy germs such as bacteria and viruses.
The CD4 count measures the number of CD4 cells in a sample of your blood, drawn by a needle from a vein in your arm. Along with other tests, the CD4 count helps determine how strong your immune system is, indicates the stage of your HIV disease, guides treatment and predicts how your disease may progress. Keeping your CD4 count high can reduce complications from HIV disease and extend your life.
How HIV affects CD4 cells
HIV targets CD4 cells by:
- Binding to the surface of CD4 cells
- Entering CD4 cells and becoming a part of them. As CD4 cells multiply to fight infection, they also make more copies of HIV
- Continuing to replicate, leading to a gradual decline of CD4 cells
HIV can destroy entire ‘families’ of CD4 cells. Then the diseases these ‘families’ were designed to fight can easily take over. That's when opportunistic infections are likely to develop.
When to have a CD4 count test
Your health care provider will recommend a CD4 count test:
- When you're first diagnosed with HIV. This is called a baseline measurement. It allows a comparison to be made against future measurements.
- About two to eight weeks after starting or changing treatment.
- Every three to six months after that (considered a reasonable time interval).
What the CD4 count test results mean
CD4 counts are reported as the number of cells in a cubic millimetre of blood. A normal CD4 count is from 500 to 1500 cells per cubic millimetre of blood. It is more important to pay attention to the pattern of results than to any one test result.
In general the HIV disease is progressing if the CD4 count is going down. This means the immune system is getting weaker and you are more likely to become unwell. In some people CD4 counts can drop dramatically, even going down to zero.
The test does not always correspond with how well you are feeling. For example some people can have high CD4 counts and feel poorly. Others can have low CD4 counts and have few complications.
If your CD4 count goes down over several months, your health care provider may recommend:
- Beginning or changing antiretroviral therapy.
- Starting preventive treatment for opportunistic infections.
NHS guidelines recommend starting on preventive antiretroviral therapy if CD4 counts are under 350, whether or not you have symptoms.
Factors that can affect your CD4 count
You should know that other factors can influence how high or low your CD4 count is.
- CD4 counts tend to be lower in the morning and higher in the evening.
- Acute illnesses such as pneumonia, influenza or herpes simplex virus infection can cause CD4 counts to go down for a while.
- If you have a vaccination or when your body starts to fight an infection, your CD4 counts can go up.
- Cancer chemotherapy can cause CD4 counts to go dramatically down.
- Fatigue and stress can also affect test results.
For these reasons:
- Work with your health care provider to ensure test conditions are the same each time.
- Have all your tests done at the same time of day.
- Wait for at least a couple of weeks after an infection or vaccination before getting a CD4 count test.