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Objective: The objective of this study was the analysis of the prevalence and type of primary resistance to antiretroviral drugs in patients diagnosed with HIV infection, and to determine the most appropriate empirical treatment to obtain a virological and immunological response., Patients and Methods: An observational analysis of patients with a de novo diagnosis of HIV infection during the period 2008-2010. Clinical, immunological and virological characteristics, including genotype analysis of resistance to antiretrovirals, were considered as independent variables. The dependent variable was an undetectable HIV viral load after six months of treatment. Data are provided as median (interquartile range) and absolute number (percentage)., Results: Seventy-three patients with a de novo diagnosis of HIV infection were included [53 males (73%); 36 (30-46) years-old; prior use of intravenous drugs: 5 patients (7%); hepatitis C virus co-infection: 13 individuals (18%)]. Ten patients (14%) showed symptoms attributable to acute HIV infection. A CD4+ T cell count lower than 350 mm(3) was detected in a 37% (n=27) of all patients. The initiation of antiretroviral therapy followed the GESIDA recommendations (no therapy: 20 patients; tenofovir+emtricitabine+efavirenz: 28 patients; abacavir+lamivudine+efavirenz: 1 patient; tenofovir+emtricitabine+protease inhibitors: 5 patients; abacavir+lamivudine+protease inhibitors: 1 patient; 18 patients were lost in the follow-up). After starting antiretroviral therapy, the resistance analyses detected the existence of primary resistance to antiretrovirals in 12.7% (confidence interval 95%: 3-22) of the patients, distributed as follows: isolated resistance to, nucleosides was detected in 2% (M184V), to nevirapine/efavirenz in 9% (K103N), and combined resistance to nucleosides and non-nucleosides in 2%; there were no cases of resistance to protease inhibitors. Consequently, antiretroviral therapy was changed in 5 (14%) out of 35 patients, attaining an undetectable HIV viral load at 6 months in all of them. The primary resistance to antiretrovirals was not related with epidemiological, virological (including infection by non B subtype) or immunological variables., Conclusions: In the present study, a change in the epidemiological pattern of de novo diagnosis of HIV infection in our area has been observed. The existence of resistance mutations in more than 5% of the new cases is noteworthy. This finding must be considered in order to establish the rules of empirical treatment in our area., (Copyright © 2011 Elsevier España, S.L. All rights reserved.)