Question: What is drug resistance testing?
Answer: Resistance testing evaluates whether HIV has evolved resistance to particular drugs. It can help a physician decide on an initial antiretroviral regimen and help guide choices when treatment needs to be changed. There are two types of resistance testing.
Genotype assays
This test examines viral genetic material and identifies particular mutations responsible for resistance. Generally the viral load needs to be above a certain value, i.e. greater than 1000 copies/mL, in order for genotype testing to be of any use. If genotype testing reveals a mutation known to cause resistance to a particular drug, then an alternative therapy and drug choice must be considered.
Phenotype assays
This test measures the ability of the virus to grow and replicate in the presence of varying concentrations of drugs. A virus that grows well in the presence of even a high concentration of drug is a sign of resistance to that drug.
