It can help a physician decide on an initial antiretroviral regimen and help guide choices when treatment needs to be changed. If genotype testing reveals a mutation known to cause resistance to a particular drug, then that person s drug therapy needs to be managed accordingly. Phenotype assays measure 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. There are many different types of both genotype and phenotype assays, most of which still need to be clinically tested and adequately standardized before becoming commercially available.
Negative results are difficult to interpret.
The assays can't detect all resistant genotypes or phenotypes. For example, there is some concern that HIV in pregnant women and their newborn children is more likely to develop resistance than in the general HIV-positive population. Patients should consult with their doctors to find out if they have an increased risk for resistance, and if so, how resistance testing would benefit them. The American Medical Association cautions that decisions to change therapy should be based primarily on a confirmed increase in viral load and the patient s treatment history. Resistance testing should be considered only as an additional source of information that may or may not be helpful.