Because of the increasing range of anti-HIV agents available, there is increasing pressure on the provision of antiviral susceptibility assays. There are two types of antiviral susceptibility assays: phenotypic and genotypic assays

Phenotypic assays define whether a particular strain of virus is sensitive or resistant to an antiviral agent by determining the concentration of the drug needed to inhibit the growth of virus in vitro. e.g. Plaque-reduction assay for HSV, plaque-reduction assay for HIV. However, phenotypic assays can only be used for viruses that can be cultivated. Moreover, in the case of HIV, plaque reduction assays may not be that appropriate since not all HIV strains produce plaques in cell culture.

In the case of genotypic assays, mutations that are associated with resistance are assayed for by molecular biology methods such as PCR and LCR.  However, these assays are tedious and are not suitable for a routine diagnostic laboratory. Moreover, he results of genotypic assays may prove very difficult to interpret since HIV  mutates at a furious pace, and it is also possible that resistant strains are present right at the beginning of infection. Despite these shortcomings, the use of genotypic assays for monitoring antiviral susceptibility has become part of the management protocol for HIV infection in many advanced countries.