HIV infection causes gradual changes in the human body. These changes, for example, in the functioning of the immune system, can be measured with various blood tests. These tests provide information about how far a person's HIV disease has progressed.

The CD4 cell count is an important test which measures immune function. Another common test, the viral load test, provides a direct measure of the amount of HIV circulating in the blood.

Monitoring viral load, immune system cells and other markers provides a person with information about the effects of HIV on his or her body. The results are interpreted in the context of knowledge about the natural history or normal course of HIV disease and AIDS.

Results may be useful tools for predicting the course of HIV infection within an individual. For example, an individual's risk of developing opportunistic infections is directly related to his or her CD4 cell count. Furthermore, the level of an individual's viral load today can help to predict how rapidly his or her CD4 cell count is likely to drop, and the risk of developing AIDS within the next few years. For example, without treatment, a person with a viral load above 55,000 copies/ml and a CD4 cell count below 200 cells/mm³ has an 85% chance of developing AIDS within three years.

The results are used to inform treatment decisions. For example, a person may decide to go on antiviral therapy after increases in viral load on two consecutive occasions. Likewise, the CD4 count indicates whether a person is at risk from specific opportunistic infections. The decision to start PCP prophylaxis is usually made on the basis of the CD4 count falling below the 200 to 250 range.

Regular monitoring and follow-up is beneficial for people with HIV both for treatment and for coping with psychological problems. A study has recently shown that knowledge of HIV infection for a long period prior to a diagnosis of AIDS is associated with improved survival compared with a short time period. However, the role and purpose of regular monitoring will vary at different stages of HIV infection.

Monitoring of treatment effect

If you do decide to start anti-HIV treatment, it is important to monitor the effects of treatment. The aim of monitoring is not only to see whether there has been a response to treatment, and how great a response, but also to monitor for side-effects or other problems.

Changes in viral load within the first few weeks of starting a new anti-HIV regimen have been shown to be a good predictor of the regimen's likely effects over the coming years. Likewise, if your viral load is increasing while you are receiving anti-HIV therapy, that indicates that the anti-HIV effects of your current treatments are waning and that it is time to consider new options.

Much more information on all these aspects of monitoring treatment is given in CD4 cell counts, Viral load and Other markers in Viral load, CD4 cell counts and other tests.

Surrogate markers

Viral load and CD4 counts are often described as surrogate markers when considering the effectiveness of medication.

Surrogacy means substitution of one thing for another. If you can't measure something, then a stand-in or surrogate measure may exist to predict the outcome.

Surrogate markers are important in anti-HIV drug trials because the effects of drugs in slowing HIV disease may not be obvious for many years. A useful surrogate marker can allow a quicker, smaller and cheaper trial to provide useful results. The prognostic markers used in HIV, such as CD4 count and viral load, are surrogate markers which may be used to predict the long-term effects of a treatment .

An ideal surrogate marker changes reliably in a certain way, allowing researchers to predict what will happen to the real measurement of interest. For example, the short-term effects of a certain drug (drug X) on a certain test measurement (measurement A) may be found also to predict the long-term effects of drug X on a different measurement (measurement B). If three months of treatment with drug X makes measurement A increase by 10%, this might also predict that drug X will increase measurement B (which could be survival over the next three years) by 15%. Therefore, if a different drug (drug Y) causes a greater increase in measurement A - say for example that three months of treatment results in a 20% increase in measurement A - then we can also predict that the effect of drug Y on measurement B - survival over 3 years - will also be greater; namely, to increase it by 30%.

It is not difficult to see why surrogate markers are attractive in HIV drug trials. For people with HIV infection there may be advantages in using surrogate markers so that treatments will become available sooner, and so that trials do not need to accrue clinical endpoints, such as infections and deaths, in order to prove the efficacy of a treatment. The disadvantage is that if markers which do not capture the full effects of the treatment are used, it may never be possible to prove what the real effects of the treatment are.