Antiretroviral resistance testing is an essential tool for the clinical management of Human Immunodeficiency Virus (HIV-1)-infected persons and HIV surveillance in the community. In the absence of appropriate treatment, HIV-1 infection leads toward the collapse of the immune system and death of most HIV-1-infected persons. Adequate antiretroviral treatment usually reverses this process. Nevertheless, HIV-1 easily develops treatment resistance through the accumulation of mutations in its genome. This causes treatment failure, and the requirement of increasingly complex, toxic and expensive treatments. Resistant viruses can be transmitted to other persons. More than 10 % of HIV-1-positive persons in Spain become infected with viruses that are already resistant to at least one antiretroviral drug. Antiretroviral treatment regimens designed on the basis of drug resistance information are more efficacious, effective and efficient.

In spite of their clinical relevance, however, conventional resistance tests are insensitive and underestimate antiretroviral resistance. By means of new ultrasensitive resistance tests it is possible to detect resistant viruses in minority populations that remain undetected by conventional genotypic tests. Ultrasensitive resistance tests thus double or triple the number of patients in whom antiretroviral resistance is detected. It is important to emphasize that detection of minority resistant mutants in antiretroviral naïve patients increases more than triples the risk of virological failure. The clinical impact of detecting minority resistant variants in treatment-experienced patients with therapeutic failure remains unknown.

Recently developed techniques of parallel emulsion sequencing of thousands of amplicon clones (Ultra deep Sequencing (UDS), Roche Diagnostics/454 Life Sciences) increase the sensibility to detect polymorphisms and mutations in highly variable genomes such as in HIV-1 in several orders of magnitude. In addition, this technique allows stablishing genetic linkage of such mutations and polymorphisms in thousands of HIV-1 clonal sequences for every patient and point of follow-up. This generates an unprecedented opportunity to characterize the nature of HIV-1 variability and the physiopathology of antiretroviral resistance in depth. Ultrasensitive resistance tests hold the promise of improving the clinical management of HIV-1 seropositive patients, avoiding unnecessary toxicities, improving epidemiological estimations of antiretroviral resistance, improving the knowledge of the pathogenesis of HIV-1 infection and antiretroviral resistance, and improving the cost-efficiency of HIV-related pharmaceutical cost.

This study aims to analyze the association between the baseline detection of resistance mutations through UDS and the virological outcome of salvage antiretroviral therapy, in comparison with conventional genotypic resistance tests. Based on the data generated in this study, new resistance interpretation tools and algorithms will be developed to improve the prediction of antiretroviral therapy outcomes. The final aim of the study is to improve the clinical care of HIV-1-infected patients through the incorporation of improved new antiretroviral resistances tests in the clinical practice.