Genotypic resistance testing examines HIV taken from a patient, looking for the presence of specific genetic mutations that are known to cause resistance to certain drugs. If a genotypic resistance test discovers a mutation at position M184V, chances are that the person's HIV is resistant to 3TC and is not likely to respond to the drug. Interpretation of these complex patterns can be difficult and incomplete in determining whether or not the virus is sensitive to particular drugs. A number of laboratories in the United States and Europe offer genotypic resistance testing. Depending on the type and number of mutations found, the researchers may be able to determine whether someone has developed resistance to a specific drug, since almost all drugs follow a set pattern of mutations.
Some drugs have single patterns of mutation, but others have complex and unpredictable patterns.
Genotypic resistance testing examines HIV taken from a patient, looking for the presence of specific genetic mutations that are known to cause resistance to certain drugs. In turn, the results may not show any drug-resistant mutations, but the mutant virus may still remain at very low numbers in the person's body and may quickly increase when therapy with the same drugs is restarted. Genotypic resistance testing has a few advantages over phenotypic testing, most notably the relative simplicity and speed with which the test can be performed. The testing can take as little as a few days to complete, and because it is less complex, it is somewhat cheaper to perform. Still, some disadvantages of genotypic resistance testing are worth noting.