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Background: The difference in type of antibiotics and its susceptibility to Bacteroides fragilis may influence warfarin anticoagulation. However, these influences have not been clarified in clinical settings.Objectives: This study aimed to investigate association between the prothrombin time-international normalized ratio (PT-INR) and concomitant use of antibiotics in real-world population of warfarin users.Methods: This was a single-center cohort study using data from health records and included patients who received β-lactams (BL)/fluoroquinolones (FQ) during ongoing warfarin treatment (2011–2015) at Hamamatsu University Hospital in Japan. Antibiotics were categorized into those which have susceptibility to Bacteroides fragilis (BLsus, FQsus) or non-susceptibility (BLnon, FQnon) and into those given orally (BLpo, FQpo) or intravenously (BLiv, FQiv). Outcomes were excessive PT-INR and changes in PT-INR, defined as the ratio (INR ratio) and difference (ΔINR) of maximum PT-INR and baseline PT-INR. Excessive PT-INR was graded as INR ratio of >1.5 or >2.5.Results: Total of 1,185 warfarin user were included. The proportion of INR ratio >2.5 in FQiv was higher than in BLiv (95% CI: 1.59-46.5). The proportions with an INR ratio of >1.5 in BLsus and FQsus were higher than in BLnon (1.72-14.1) and FQnon (1.05-9.36), respectively. ΔINR values in FQpo, FQiv and FQsus were higher than those in BLpo, BLiv and FQnon, respectively. Conclusions and Relevance: Concomitant use of fluoroquinolones or of antibiotics which are susceptible to Bacteroides fragilis is associated with higher risk of excessive anticoagulation. This findings would contribute to safe and proper antibiotic treatment in warfarin user., 雑誌掲載タイトル: Association Between the Prothrombin Time–International Normalized Ratio and Concomitant Use of Antibiotics in Warfarin Users: Focus on Type of Antibiotic and Susceptibility of Bacteroides fragilis to Antibiotics