testing looks for the presence of changes in HIV's genetic structure that are commonly associated with the development of drug resistance. When present, these changes, called mutations, alter the way the virus makes key proteins (like reverse transcriptase and protease) and often cause resistance to specific drugs. Phenotypic testing is a more direct measure of resistance. It examines the amount of drug needed to inhibit the growth of a person's HIV in a laboratory setting.
In its natural or wild type state, HIV is not resistant to any particular drug and known drug levels are needed to suppress HIV replication. Drug-resistant HIV requires higher levels of the same drug to remain suppressed. However, because few if any direct comparisons have been made between the two types of tests, at this time no single test was determined better than any other. Both genotyping and phenotyping are useful and may be used differently in various settings (treatment management vs. drug development, etc.).
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