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When and why

This test, alongside the viral load test, is an essential part of both the monitoring of the course of HIV infection over time and the response to treatment. In someone who is well and symptom-free it is usually done two to four times a year, but in someone who is symptomatic it may be carried out more frequently to assess both the risk of opportunistic infections and response to anti-HIV drug treatments.

Among the body's white blood cells (or leukocytes) is a subgroup called the lymphocytes. CD4 T-cells are a type of lymphocyte that co-ordinate the immune system's response to certain micro-organisms such as viruses. HIV can infect and kill CD4 T-cells, as well as some other types of cell.

The test, performed on a small sample of blood taken from a vein in the arm, calculates the number of CD4 T-cells per cubic millimetre of blood. The test measures the number of CD4 T-cells circulating in the blood, which is approximately only 2% of their total number in the body. The majority of CD4 T-cells are to be found in the lymph nodes.

The absolute CD4 cell count measures the number of CD4 T-cells in each cubic millimetre of blood. A normal count in a healthy, HIV-negative adult can vary but is usually between 500 and 1500 cells/mm3. In very young children the normal CD4 cell count is much higher.

CD4 T-cells are not the only type of lymphocyte. Another type are CD8 T-cells, which kill abnormal or infected body cells. Instead of counting the number of CD4 T-cells per cubic millimetre, doctors sometimes assess what proportion of all the lymphocytes are CD4 T-cells. This is called the CD4 percentage; in HIV-negative people, a normal result is about 40%. A CD4 percentage below about 20% is thought to reflect a risk of opportunistic infections about the same as an absolute CD4 count of about 200 cells/mm3. Some doctors argue that this is potentially the most accurate CD4 test, although it is not very sensitive to small changes.

A third approach is called the CD4:CD8 ratio, in which the number of CD4 cells in a sample of blood is compared with the number of CD8 cells. The result is given as a single figure, which indicates how many CD4 cells are present for each CD8 cell. A normal result is greater than 1 (i.e. there is at least one CD4 cell for every CD8 cell in the sample), but this tends to fall to below 1 if HIV disease progresses. Both the CD4 percentage and the CD4:CD8 ratio are also affected by changes in the number of CD8 cells, which tends to rise through the course of HIV infection.

How it will help

The absolute CD4 cell count is the main test that doctors use to monitor your immune system. Most people with HIV find that over time their CD4 cell count falls, although there may be long periods when it remains very stable. If it falls below certain levels, you are potentially at risk from certain opportunistic infections, so you may be offered treatments to try to prevent them. Likewise, monitoring your CD4 count can help you decide whether to start taking anti-HIV drugs, to try to prevent any further damage to your immune system.

If you are taking anti-HIV drugs, the trend in your CD4 cell count may help to show how well the treatment is working. A steady increase in your CD4 cell count after starting treatment is a good sign; if your CD4 cell count is below 200 cells/mm3 when you start treatment, monitoring your CD4 count will pinpoint when it is possible for you to stop taking prophylactic or maintenance treatments for opportunistic infections. Ongoing monitoring of the CD4 cell count will also provide you with useful information about the safety of stopping treatment, and when it might be advisable for you to re-start treatment. If your CD4 cell count was below 200 when you started treatment, it is likely to return to this level within six months of stopping.

The frequency at which you are advised to have a CD4 cell count will depend on the current state of your immune system, and whether or not you are taking anti-HIV drugs. Among untreated people whose CD4 counts are above 500, tests are usually performed only every six to twelve months. At CD4 counts between 350 and 500, tests are likely to be performed about every three to six months, or maybe more often if recent tests suggest that the CD4 count is falling. People with counts between 250 and 350 may be offered a CD4 count more often, so that precautions such as prophylaxis against PCP (pneumonia) can be suggested if the count falls below 200.

If you are starting on anti-HIV drugs, your doctor is likely to recommend a CD4 count before you start treatment, followed by a second test after four to eight weeks to assess the immediate impact of the drugs. Thereafter, a CD4 count will probably be done between monthly and three-monthly.

It is crucial to follow the changes in the count over a series of tests, rather than to worry too much about an individual result. HIV isn't the only thing that can affect your CD4 count. Other infections, the time of day, whether or not you smoke and your stress levels can all have an impact on the test result. In addition, some people can remain well despite having a low CD4 count and others can develop symptoms and problems despite having a higher CD4 count.

Secondly, a CD4 test only indicates the number of CD4 cells in the blood. Most of the CD4 cells in the body are in tissues such as lymph nodes, rather than in the blood. Various factors can encourage CD4 cells to move into or out of the blood which would show up as a higher or lower CD4 cell count respectively, even though the total number of CD4 cells in your body remained unchanged. Again, this highlights the importance of watching the general trend, rather than a single result.