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Introduction: Warfarin is a narrow therapeutic index drug which is needed a monitor to maintain INR within the target range for optimal efficacy and safety. Due to its action could be affected by many factors, INR target is sometimes difficult to control. This analytical, cross-sectional study was designed to determine factors affecting INR control and bleeding in warfarin-treated patients. Methods: Patients who visited the warfarin clinic at Warinchamrab hospital during May 2015 to May 2016 were enrolled. Patient data were collected from medical records including demographic, social history, medical history, and pharmacogenetics testing. Data were analyzed with descriptive statistic and logistic regression analysis. Results: Sixty patients were included into the study. Atrial fibrillation with valvular heart disease was the most indication of warfarin with 46.7%. The INR target for most patients (83.3%) was between 2.0 and 3.0. Percentage of patients achieving TTR over 60% was 36.7%. The results from multivariate logistic regression analysis showed that factor related to TTR under 60% was polypharmacy (OR 50.156, p 0.031). Patient who had bleeding showed that the ratio of female patient compared with male patient were 38.7% and 10.3%, respectively (OR 5.474, p 0.011). The ratio of patient who had bleeding in patient with GFR <60 and >60 mL/min/1.73m2 were 46.7% and 18.6%, respectively (OR 3.828, p 0.033). Conclusion: Polypharmacy was the significant factor related to INR out of target, whereas female and GFR <60 mL/min/1.73m2 could be risk factors for bleeding in patient receiving warfarin. The results could be applied to improve in INR control of warfarin clinic.
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