While viral load tests can help patients and doctors determine whether or not a treatment is effective, drug resistance tests may help determine why a treatment or combination of treatments may not be working. Drug resistance tests may also be useful in choosing treatments. Genotypic assays are really nothing more than careful inspections of the HIV in someone's blood. While some mutations are harmless, others may cause HIV to become less sensitive to a drug designed to stop it from dividing. Over time, this HIV accumulates additional mutations which prevent it from responding to the drug altogether.
Genotypic tests look for the key mutations associated with resistance to a particular drug.
By finding out which mutations are present, it may be possible for patients and their doctors to figure out which treatments they should start with and, if necessary, which treatments they should switch to. Unlike genotypic testing, phenotypic assays do not look at the actual genes or RNA of the virus but instead look at the ability of the virus to replicate in a test tube when a particular drug is present. Test results are given as the amount of drug necessary to suppress the virus. HIV that is not resistant will be completely suppressed by appropriate levels of drug. Resistant HIV requires adding considerably more of the drug to completely suppress the virus.