When and why

How it will help

The viral load is thought to give an indication of the rate at which HIV is reproducing in the body. In people who are not taking anti-HIV drugs, it predicts an individual's risk of developing AIDS-related illnesses within the next three years. As with the CD4 cell count, following the trend in your viral load count over time may help you decide when to think about starting anti-HIV treatment.

If you do decide to start treatment, testing your viral load beforehand (at baseline) and again after you have been taking the drugs for one to two months may indicate how well they are working. Effective anti-HIV regimens rapidly reduce the HIV viral load in the blood. Several clinical endpoint trials have suggested that the reduction in viral load two to three months after treatment begins is likely to predict how effective a treatment is at delaying the development of symptoms and/or prolonging life. If a new treatment regimen does not have a significant impact on your viral load, many doctors would recommend that you try a different combination instead.

In some cases, combinations can reduce plasma viral load to below the limits of detection, even among people who have low CD4 counts or who have taken anti-HIV drugs before. If your viral load is 'undetectable', HIV may be less likely to develop resistance to the drugs. However, the drugs may not have such a powerful effect throughout the body, and the effect may not last. There is no evidence that people with 'undetectable' viral load are no longer infectious to others. See Treating HIV in the genital tract in Anti-HIV therapy: Choosing your treatment strategy for further discussion of this issue.

If you are taking anti-HIV drugs correctly but your viral load starts to rise again or returns to its baseline level, this probably indicates that the drugs' antiviral effects are waning, perhaps due to resistance. However, doctors differ in the extent to which they recommend basing treatment decisions only on viral load results. Some argue that the aim of treatment should be to achieve and maintain undetectable viral load. Others are concerned that with today's drugs this is unachievable for many people and will encourage them to change drugs too rapidly, until eventually they run out of treatment options.

As with CD4 counts and other laboratory tests, what matters most may be the trend in viral load over time, rather than any one single test result. The same viral load test used on the same sample of blood can produce a different result because of the degree of variability in the test. Various factors can cause a temporary blip in the viral load, especially things that stimulate the immune system such as vaccinations. People who develop an opportunistic infection tend to experience a temporary increase in viral load of around 1 log, which usually returns to its previous within a couple of months of the successful treatment of the infection.

For all these reasons, some researchers argue that having your viral load measured should ideally consist of two tests on different samples of blood taken a few days apart, especially if the result may prompt important medical decisions.