Drug resistance testing is a critical tool for assisting healthcare professionals when selecting antiviral drug (ARV) regimens for people living with HIV. Clinicians must gather all the information possible, including patient history, ARV history, CD4 count, viral load and past and current resistance tests to select an optimized background therapy.

Drug resistance testing allows clinicians to evaluate their patient’s resistance profile with confidence by showing which drugs are susceptible, partially sensitive or resistant. Current resistance tests include a genotype, phenotype or combination test, which includes both a phenotype and a genotype.

Combined HIV Genotypic and Phenotypic Testing

Combined phenotypic and genotypic tests provides the complete picture of resistance and will allow the clinician to construct an optimal antiretroviral regimen. Current DHHS guidelines recommend that having both tests (genotype and phenotype) might provide critical and complementary information to guide regimen changes for patients with a complex treatment history.

HIV Phenotypic Testing

Phenotypic testing is one of two types of HIV drug resistance testing. Phenotypic testing is performed by exposing a sample of an individual's HIV to all of the available antiviral drugs. By directly measuring the ability of HIV to grow in the presence of these drugs, scientists can determine which drugs will work and which are no longer good options. The activity of a person's HIV in the presence of the antiviral drugs is compared to the activity of a control strain of HIV that is known to be susceptible to a specific drug. This comparison can determine how likely or unlikely a person is to respond to that drug.

The results of phenotypic testing can be used by a healthcare team to select the most active antiretroviral regimen for a particular individual.

HIV Genotypic Testing

Genotypic testing is the other of the two types of HIV drug resistance testing. Genotypic testing is performed by identifying a pattern of mutations in a sample of an individual’s HIV. These specific, genetic patterns are associated with resistance to specific antiretrovirals providing an indirect measure of susceptibility.