In the majority of patients who have never received antiretroviral therapy, the wild type, or non-mutant virus predominates. During therapy the disappearance or suppression of wild-type virus creates the environment in which the mutant virus can become the dominant species. The degree of suppression provided by a treatment regime is therefore, a critical factor in the emergence of HIV drug resistance. Resistance assays may assist clinicians in individualizing initial as well as subsequent antiretroviral treatment regimens for their patients. Resistance testing is recommended for persons on anti-retroviral treatment whose viral is increasing and CD4 cell count is declining.

Many studies in patients on treatment have shown strong associations between the presence of drug resistance and failure of the antiretroviral therapy. There are mainly 2 types of resistance testing which unfortunately are not yet readily available in Kenya.

Genotypic Assays

These detect drug resistance mutations that are present in the relevant viral genes. They may involve sequencing of the entire RI and Protease genes while others go for selected mutations that are known to confer drug resistance.

Phenotyping Assays

These assays measure the ability of viruses to grow in various concentrations of antiretroviral drugs. Although available they are more expensive and time consuming to perform.

In general resistance testing may be useful in the setting of virologic failure of antiretroviral therapy or in acute HIV infection.