What is a CD4 count?

A CD4 count is the result of a blood test. It tells you how many CD4 cells are in a cubic millimetre of blood (mm³). Information about units of measure for blood tests.

The full medical name is a CD4+ T lymphocyte count. It is also called CD4+ T cell or T4 count.

Everyone should have a CD4 count soon after they are diagnosed with HIV. The result is used, with other factors, to decide when to start treatment. Guidelines recommend starting treatment at different CD4 counts cut offs depending on which country you live in.

• All guidelines recommend that treatment should be started when someone has a CD4 count that is less than 200.
• Most guidelines (including the UK and the WHO), treatment is recommended when the CD4 count is around 350 cells/mm³.
• Some guidelines over the last year raised this threshold to 500.
• Guidelines in the US include an option that treatment could be started at any CD4 count over 500.
• Currently, it is unclear whether starting above 500 has any long-term clinical advantage over waiting until under 500 or under 350. Some studies report no difference, other suggest there may be a small difference.

CD4 counts should be tested every three months. Sometimes lack of resources mean it may be done every six months.

What is a surrogate marker?

A surrogate marker is when something that is easy to measure directly is used to measure something else indirectly.

The CD4 count is a good surrogate marker for how much HIV has damaged your immune system. It can tell you your risk of infections and when you need to start treatment. It doesn’t directly measure HIV.

The normal range for a CD4 count in an HIV-negative person is between 400 and 1,600. This range includes 95% of people but 5% of HIV-negative people have levels that are either naturally lower or higher levels than this.

Pattern of CD4 count after HIV infection

• A few weeks after infection with HIV the CD4 count falls.
• Then as the body’s immune system begins to fight back, it goes back up again – though not to the levels that it was before HIV infection.
• This level is sometimes called the CD4 set point and usually takes about 3- 6 months after infection to stabilise, but it can take much longer.
• Then the trend for the CD4 count is to gradually go down over several years. The average rate that CD4 counts fall is about 50 cells/mm³ every year. In some people CD4 counts will fall much faster and in some people much slower.

Most people’s immune system controls HIV very successfully without needing HIV drugs for many years.