There is growing evidence of the utility of diagnostic testing to direct treatment of HIV patients with resistant viral strains. Which test to use, however, is complicated by conflicting data regarding whether genotyping assays or phenotyping assays provide more clinically useful information.

HIV genotyping entails sequencing virus isolated from a patient and looking for mutations known to confer resistance to a given therapeutic. By contrast, HIV phenotyping involves taking virus from a patient and growing it in culture so that the virus' resistance to any antiviral added to the culture may be measured.