HIV infection gradually causes changes in the human body. The level at which the immune system is functioning can be monitored with several blood tests that provide insight into both how well someone's immune system is fighting the virus and the type and amount of virus present.

The CD4 cell count measures immune function. Another common test, the viral load assay (measuring HIV-1 RNA), provides a direct measure of the amount of HIV circulating in the blood. Both of these tests are considered independent markers of HIV disease progression.^[1]

A viral load assay and CD4 cell count together can:

• Evaluate current immune system status.
• Guide and monitor treatment decisions.
• Predict rate of disease progression in both the short- and the long-term.

CD4 cells counts, viral load assays, and resistance testing provide information about the effects of HIV disease on an individual that can then be interpreted in the context of what is known about the natural history, or normal course, of HIV disease and AIDS.

Viral load assays and CD4 cell counts are used to inform treatment decisions. A person may be advised to start antiretroviral therapy (ART) when the CD4 cell count declines below a certain level. In some places, the use of antiretroviral treatment (ART) is not authorised and/or its cost reimbursed by insurance until the CD4 count falls below a certain level.

The decision to start prophylaxis against certain pathogens is often based on the CD4 cell count number, as the risk of developing certain infections is directly related to the CD4 cell count. In places where CD4 testing is not readily available or affordable, prophylaxis may be offered based on clinical impression and/or a pathogen's prevalence in a particular setting.

The decision to start ARV therapy is often made when a person has detectable viral load. A viral load decrease within the first few weeks of starting an ARV regimen is generally a positive predictor of regimen success over time. Subsequent viral load increases on two consecutive occasions may indicate a need to change the ARV regimen or to take a closer look at adherence. Even a single HIV-1 RNA count is highly predictive of time to AIDS, but a series of measurements over time better predicts disease progression.^[2][3]

Resistance testing can provide information on drugs that a particular HIV virus may be sensitive or resistant to (phenotypic testing) and on mutations in the genetic code of the HIV virus (genotypic testing) that would affect treatment choices.