OBJECTIVES: To describe regional differences and trends in resistance testing among individuals experiencing virological failure and the prevalence of detected resistance among those individuals who had a genotypic resistance test done following virological failure. DESIGN: Multinational cohort study. METHODS: Individuals in EuroSIDA with virological failure (>1 RNA measurement >500 on ART after >6 months on ART) after 1997 were included. Adjusted odds ratios (aORs) for resistance testing following virological failure and aORs for the detection of resistance among those who had a test were calculated using logistic regression with generalized estimating equations. RESULTS: Compared to 74.2% of ART-experienced individuals in 1997, only 5.1% showed evidence of virological failure in 2012. The odds of resistance testing declined after 2004 (global P < 0.001). Resistance was detected in 77.9% of the tests, NRTI resistance being most common (70.3%), followed by NNRTI (51.6%) and protease inhibitor (46.1%) resistance. The odds of detecting resistance were lower in tests done in 1997-1998, 1999-2000 and 2009-2010, compared to those carried out in 2003-2004 (global P < 0.001). Resistance testing was less common in Eastern Europe [aOR 0.72, 95% confidence interval (CI) 0.55-0.94] compared to Southern Europe, whereas the detection of resistance given that a test was done was less common in Northern (aOR 0.29, 95% CI 0.21-0.39) and Central Eastern (aOR 0.47, 95% CI 0.29-0.76) Europe, compared to Southern Europe. CONCLUSIONS: Despite a concurrent decline in virological failure and testing, drug resistance was commonly detected. This suggests a selective approach to resistance testing. The regional differences identified indicate that policy aiming to minimize the emergence of resistance is of particular relevance in some European regions, notably in the countries in Eastern Europe.

Department of Infection and Population Health University College London London UK bFundacions irsiCaixa i Lluita contra la SIDA Hospital Universitari Germans Trias i Pujol Universitat Autònoma de Barcelona Badalona Catalonia Spain cDivision of Infectious Diseases and Hospital Epidemiology Departments of Medicine and Clinical Research University Hospital Basel Switzerland dUniversitäts Klinik Bonn Germany eDepartment of Infectious Diseases 3rd Faculty of Medicine Charles University and Hospital Na Bulovce Prague Czech Republic fClinic for Infectious Diseases University Medical Centre Ljubljana Slovenia gService des Maladies Infectieuses APHP Hôpital St Antoine and INSERM UMR_S1136 Paris France hState Agency Infectology Center of Latvia Riga Latvia iRigshospitalet University of Copenhagen CHIP at Department of Infectious Diseases Copenhagen Denmark

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