Antiretroviral drug resistance and resistance testing.

Antiretroviral resistance testing should be performed in newly diagnosed patients with acute or recent HIV infection and at the time of treatment failure, and there is growing support for testing in newly diagnosed, treatment-naive patients with chronic infection as well. Genotypic testing is preferred for baseline screening, because it is more sensitive than phenotypic testing for the presence of mixed populations of drug-susceptible and -resistant virus and because it is less expensive. Phenotypic testing provides quantitative information on the degree of resistance and is also able to assess interactions among mutations. As a result, it can be particularly useful in determining treatment options for treatment-experienced patients with multi-drug resistant virus. In many cases, there may be advantages to the use of both tests.