Evaluation of time in therapeutic range among patients receiving warfarin therapy: A retrospective cohort study at one private hospital in Thailand

The quality of warfarin therapy is often measured by the percentage of time that a patient spends within target international normalized ratio (INR) range (time in therapeutic range, TTR). It has been found that TTR can strongly predict both bleeding and thromboembolic events. This retrospective cohort study was conducted to evaluate the mean TTR and the predictors of inadequate anticoagulation control among subjects attending the warfarin clinic at one private hospital in Thailand during June 2012 and May 2016. Study subjects consisted of patients who had been taking warfarin for all indications with target INR 2.0-3.0. TTR was calculated through the Rosendaal method which provides the percentage of days when INR values are in desired range. A total of 196 patients (71.9% female, mean age 69.55 years) recruited represented the average TTR value of 60.46%. The stratification of patients according to anticoagulant control levels indicated that the poor control group (TTR < 65%) and the good control group (TTR 65%) contained 103 patients (52.55%) and 93 patients (47.45%), respectively. The mean TTR value of the poor control group was significantly lower than the good control group (43.64% vs. 79.09%; P < 0.001). It was found that comorbid heart failure, history of non-adherence, warfarin-drug interaction and warfarin-food/herb interaction were associated with the status of poor anticoagulant control (adjusted OR were 7. 258, 18.232, 2.886 and 5.828, respectively). Recognition of these predictive factors could be beneficial in improving pharmaceutical care activities in order to optimize TTR value among patients receiving warfarin therapy.