Testing for Resistance

Healthcare providers (HCPs) can do genotypic testing and/or phenotypic testing to help detect resistance to antiretroviral drugs.

Genotypic testing
Genotypic tests look for changes in HIV that are associated with drug resistance. These tests help your HCP find out if and how your virus has changed and which drugs are not likely to work.

Genotyping can be used to help determine the potential for drug resistance by looking at the reverse transcriptase and protease enzymes of a person’s virus.

A single mutation may mean resistance to more than one drug or an entire drug class.

The genotypic testing process
Step 1—A blood sample is taken.
Step 2—Copies of the virus are made from the blood sample.
Step 3—Genes within the virus are evaluated for mutations.
Step 4—Mutations found in the virus are compared with known resistance mutations.

If the type and number of mutations present in a person’s HIV match pre-established mutation patterns for a particular drug, that individual’s virus has probably developed resistance to that drug.

If genotypic testing reveals drug resistance mutations in a person’s HIV, certain antiviral drugs may be less likely to work for him or her. The specific type and placement of the mutations dictate to which drugs the virus may be resistant. Once an HCP understands which drugs are less likely to work for his or her patient, he or she can develop a treatment plan that avoids the use of those drugs. This results in a therapy regimen that is more likely to be effective for a longer period of time.

Phenotypic testing
Phenotypic tests directly measure the amount of resistance that HIV has to each drug. This test is done for most HIV medicines you are taking, or may take, to learn which drugs may work well for you.

Phenotypic testing directly exposes a person’s virus to many of the currently available antiviral drugs. The virus is exposed to a specific drug and then evaluated to determine how much of the drug is required to block viral activity.

The phenotypic testing process
The phenotypic testing process can be summarized in two key phases: making the HIV copy and measuring phenotypic resistance.

Making the HIV copy
Step 1—A sample of blood is taken from the patient.
Step 2—From the sample, key portions of the genes in the patient’s virus are copied.
Step 3—The copied genes are inserted into a laboratory sample of HIV.
Step 4—The laboratory HIV becomes a genetic copy of the patient’s HIV.

After a copy of the HIV is made, phenotypic resistance can be measured.

Measuring phenotypic resistance
Step 1—The copies of the patient’s virus made in the previous steps are exposed to antiretroviral drugs.
Step 2—The ability of the virus to grow (or not grow) in the presence of each antiviral drug is evaluated.
Step 3—The results help HCPs determine how much of a particular drug is needed to stop the patient’s HIV from reproducing.

Once an HCP has a complete understanding of a person's resistance status and therapy options, he or she can develop a treatment plan that avoids the use of ineffective drugs. This results in a therapy regimen that is more likely to be effective for a longer period of time.