Current levels of drug resistance among therapy-naive HIV-infected patients have significant impact on treatment response.

Resistant HIV can be transmitted from one patient t o another. 1 Recent large-scale European research found onetenth of viruses from untreated patients showing at least 1 resistance-related mutation. 2 Such investigations are mainly driven by the concern that resistant virus c ould at some point hamper optimal treatment response. However, the impact of baseline drug resistance on treatment response is not well studied. Various gen otypic interpretation methods are used to assess resistanc e in drug-naive patients. When transmitted resistan ce is the topic of interest, irrespective of treatment, prima ry mutations are considered most indicative. Second ary mutations could also be the result of natural varia tion. However, when interpreting resistance in view of the response to the installed treatment, all positions possibly contributing to the selective advantage of the virus in the presence of drug should be taken into account.