HIV-1 genotypic drug resistance in patients with virological failure to single-tablet antiretroviral regimens in southern Taiwan Hung-Chin Tsai,1–3 I-Tzu Chen,1 Susan Shin-Jung Lee,1,2 Yao-Shen Chen1,2 1Division of Infectious Diseases, Department of Medicine, Kaohsiung Veterans General Hospital, Kaohsiung, Taiwan; 2Faculty of Medicine, School of Medicine, National Yang-Ming University, Taipei, Taiwan; 3Department of Parasitology, Kaohsiung Medical University, Kaohsiung, Taiwan Purpose: Sparse data are available on the prevalence of resistance among HIV-1-infected patients with virological failure to a single-tablet regimen (STR). This study aimed to evaluate the prevalence of HIV genotypic drug resistance in HIV-1-infected patients with virological failure to STRs in southern Taiwan. Patients and methods: This retrospective study investigated drug resistance in patients with virological failure to STR from January 2016 to September 2017. Antiretroviral resistance mutations were defined using the 2017 International AIDS Society-USA HIV drug resistance algorithm, and drug resistance was compared using the HIVdb program of the Stanford University HIV Drug Resistance Database. Variables between resistance and non-resistance groups were compared. Results: Thirty-nine HIV-1-infected patients with treatment failure were tested for resistance, of whom 89% were infected by men who have sex with men. Subtype B HIV-1 strains were found in 90% of the patients. Eight patients were treatment naïve and initiated STRs, while 31 patients experienced treatment failure after switching to STRs. Eighty-seven percent of the patients harbored any of four classes of resistance (nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors (PIs), and integrase strand transfer inhibitors). The prevalence rates of nucleoside reverse transcriptase inhibitor, non-nucleoside reverse transcriptase inhibitor, PI, and integrase strand transfer inhibitor resistance were 72%, 82%, 10%, and 3%, respectively. Patients with PI resistance were more likely to respond to treatment with a non-tenofovir disoproxil fumarate/emtricitabine/efavirenz-based STR (P