1 Immune system and CD4 count

1.7 CD4 count as a surrogate marker

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What is a CD4 count?

A CD4 count is the result of a blood test that tells you how many of these cells are in a cubic millimetre of blood (mm³).

Full name: CD4+ T lymphocyte count, but also called CD4+ T cell or T4 count.

Everyone should have a CD4 count soon after they are diagnosed with HIV. If this is under 200 cells/mm³ it is usually recommended to start treatment. CD4 monitoring should be done 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.

The average CD4 count for an HIV-negative person is usually between 600 and 1,600 – but a few people have levels that are 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.