Hooi Sun Ooi

Abstract

Warfarin is a vitamin K antagonist (VKA) with a narrow therapeutic window prescribed to prevent thromboembolic events. Time in therapeutic range (TTR) is the most recognised way to measure anticoagulation quality over time. High TTR correlates with reduced thromboembolic events and low TTR with increased complications. Objective: To evaluate anticoagulation quality for patients receiving warfarin at Queen Elizabeth Hospital. Method: A retrospective observational population study was conducted on patients reviewed under the Warfarin Medication Therapy Adherence Clinic (MTAC) from April 2016 to April 2017. Data was collected using a data collection form. Patients who were on warfarin treatment for at least 6 months or at least 5 visits for those newly started with warfarin were included in the study. The primary outcome was the time in therapeutic range (TTR) of 1.8-3.2, calculated using the Rosendaal method. Results: 150 patients on warfarin with 1765 clinic visits were evaluated. Most were male (56%), and the mean age was 61.33±13.47. The prevalent indication observed was nonvalvular atrial fibrillation, which accounted for 110 patients (73.3%) of the total. The overall mean TTR (1.8-3.2) was 80.6±20.2%. The wrong/missed warfarin dose was the main reason the International Normalised Ratio (INR) was out of range (150/851 episodes, 17.6%). Hospitalisation rendered most patients defaulting clinic visits (10/34 episodes, 29.4%) and was predominantly non-warfarin related (27/37 episodes, 19 patients). There was a single episode of major bleeding (1 patient). Conclusion: A high average TTR was achieved in patients taking warfarin at our institution. Wrong/missed doses and inconsistent diet were the main reasons for INR being out of range. However, the primary reason for INR being out of range remains unknown. A total of 37 cases of hospitalisation were reported, and 72.9% of them were non-warfarin related. No mortality was recorded.