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Objectives: The aim of the study was to identify variables that can influence atazanavir plasma concentration., Methods: We retrospectively analysed atazanavir trough concentration of HIV infected patients who performed therapeutic drug monitoring between October 2007 and July 2011. Qualitative variables were compared with X(2) test while continuous ones with Mann-Whitney and Student's t-test. A linear regression model was used to investigate factors influencing atazanavir plasma concentration. Therefore, we analysed the impact of cirrhosis on atazanavir pharmacokinetic variability., Results: 255 plasma samples from 179 patients were analysed. At the univariate analysis female gender (+144.4 ng/mL; p=0.05) and tenofovir (+196.8 ng/mL; p=0.002) were associated with higher atazanavir concentrations. The multivariate model confirmed these two variables (+164.6 ng/mL; p=0.02 and +150.4 ng/mL; p=0.01) as independently associated with higher atazanavir trough concentration. The analysis of cirrhotic population showed an influence of tenofovir (-255.9 ng/mL; p=0.01), increased AST (+95.3 ng/mL; p=0.09), ALT (+67.9 ng/mL; p=0.07) and creatinine (+517.2 ng/mL; p=0.04). The multivariate model confirm that tenofovir was associated with lower atazanavir trough concentration (-284.1 ng/mL; p=0.005) while AST values significantly increased atazanavir concentrations (+114.5 ng/mL; p=0.03)., Discussion: Atazanavir is a safe and manageable drug. Our results suggest that female patients tend to have higher atazanavir plasma concentration, while the effect of tenofovir needs to be better clarified.