Several retrospective studies, using genotype or phenotype assays, have shown that pre-treatment resistance testing is significantly predictive of the virological outcome to a new antiretroviral regimen. The clinical utility of resistance testing has been evaluated in randomized prospective studies which analysed whether the selection of a new regimen was improved by phenotype or/and genotype assays. These trials, including different patient populations, showed a variable but consistent short-term virological benefit in the use of resistance assays. Resistance testing is now recommended as part of care by several european and international groups of experts for
optimizing the choice of antiretroviral therapy. Genotyping that refers to the identification of the resistance mutations is more widely used that phenotyping. As regards the genotype the most important challenge is to determine the profile of mutations predicting the virological response to each available antiretroviral drug. Genotype interpretation is usually based on rules-based algorithms. Different interpretation systems exist with substantial variability in performances. The clinically relevant genotype interpretation systems are based on correlation study analyzing the profile of mutations at baseline and the virological response in treated patients.
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