CD4 cell counts

CD4 cells, sometimes also called T-cells, or T-helper cells, are white blood cells which organise the immune system’s response to bacterial, fungal and viral infections.

A normal CD4 cell count in an HIV-negative man is between 400 and 1600 per cubic millimetre of blood (but doctors normally just give a figure, e.g. 500). CD4 cell counts in HIV-negative women tend to be a little higher, between 500 and 1700.

Even if you don’t have HIV, many factors can affect your CD4 cell count. For example it’s known that:

• Women have higher CD4 cell counts than men (by about 100).

• Women’s CD4 cell counts go up and down during the menstrual cycle.

• Oral contraceptives can lower a woman’s CD4 cell count.

• Smokers tend to have higher CD4 cell counts (by about 140).

• CD4 cell counts fall after rest – by as much as 40%.

• A good night’s sleep can mean that you have a lower CD4 cell count the following morning, but a higher CD4 cell count the next afternoon.

None of these factors seems to make any difference to how able your immune system is to fight infections.

Only a small portion of your body's CD4 cells are in the blood. The rest are in the lymph nodes and tissue, and the fluctuations noted above might be due to the movement of CD4 cells between blood and tissue.

Soon after infection with HIV,  your CD4 cell count probably dropped sharply, before stabilising at around 500 to 600. It seems that people who experience a greater initial drop in CD4 cell count and a lower stabilisation in their CD4 cell count may be at risk of faster disease progression. Even while you are well and have no obvious symptoms of HIV, millions of CD4 cells are infected by HIV and lost every day, and millions more are produced to replace them.

It’s estimated, however, that without treatment, an HIV-positive person’s CD4 cell count drops by about 45 cells every six months, with greater falls experienced by people with higher CD4 cell counts.

A CD4 cell count between 200 and 500 indicates that some damage to your immune system has occurred.

Steeper falls in CD4 cell counts are experienced in the year before AIDS develops, which is why you are recommended to have your CD4 cell count monitored with increased frequency (every three months or so) once it goes below 350.

Looking at your CD4 cell count can also provide a guide for decisions about your need to start taking HIV treatment as well as other medicines to prevent some AIDS-defining illnesses. It is recommended that everybody with a CD4 cell count of 350 cells or below should start HIV treatment (this recommendation was changed in 2008 - previously the recommendation was to start treatment once the CD4 count fell to 200, but better results and less illness are seen in people who start treatment at CD4 cell counts of around 350). What's more if your CD4 cell count is below 200, you are recommended to take antibiotics to prevent you getting PCP pneumonia.

Your CD4 cell count can naturally fluctuate, so don’t put too much emphasis on a single test result. Rather, look at the trend in a number of recent CD4 cell counts.

If your CD4 cell count is high, you have no symptoms, and are not taking anti-HIV medication, then it probably only needs monitoring every few months or so.

However, if it’s falling rapidly, you are unwell, are taking part in a clinical trial, or are taking anti-HIV drugs, then it should be monitored more often.

Sometimes, as well as counting the number of CD4 cells, doctors will also assess what percentage of all your white blood cells are CD4 cells. This is called the CD4 cell percentage. A normal result in a person with an intact immune system is about 40%, and a CD4 cell percentage below 20% indicates the same risk of becoming ill with an AIDS-defining illness as a CD4 cell count of about 200.

Your CD4 cell count can be used to help decide when you need to start HIV treatment, and as an indication of how successful these treatments are.

British HIV treatment guidelines issued in 2008 recommend the following:

Once your CD4 cell count falls below 350, you should start taking HIV treatment as soon as you are ready.

If your CD4 cell count is below 250, you should start HIV treatment straight away. A CD4 cell count of below 200 indicates that you are at a real risk of becoming ill with an AIDS-defining illness.

Previous treatment guidelines recommended starting treatment when the CD4 count fell to 200. However, it seems that if you start treatment when your CD4 count is around 350, you are more likely to respond well to HIV treatment. What's more, starting treatment at the 350 level reduces the long-term risk of some serious, non-HIV-related illnesses like heart, liver and kidney disease. It is now thought that newer anti-HIV drugs are sufficiently powerful and safe to make it advisable to start HIV treatment at this time.

Once you start HIV treatment, your CD4 cell count should start to slowly increase. If you experience a fall in your CD4 cell count over a number of tests, this should alert your doctor that there may be something wrong with your HIV treatment.