Time in therapeutic range and stability over time for warfarin users in clinical practice: a retrospective cohort study using linked routinely collected health data in Alberta, Canada

OBJECTIVES: Whether warfarin-treated patients with non-valvular atrial fibrillation (NVAF) who exhibit good control will experience deterioration in control over time is uncertain. We designed this study to examine the time in therapeutic range (TTR) in a population-based cohort of patients with NVAF recently initiated on warfarin. DESIGN: Retrospective cohort study using routinely collected health data from 2008 to 2015. SETTING: The Canadian province of Alberta. PARTICIPANTS: All adults with NVAF who were taking warfarin for >1 month. MAIN OUTCOME MEASURES: Frequency of international normalised ratio (INR) monitoring and the Rosendaal TTR with time zero set at 31 days after the first warfarin dispensation. RESULTS: Of 57 669 patients with NVAF dispensed warfarin for >1 month, 17 099 (29.7%) had <3 INRs measured in months 1–6. Of the 40 570 who went for regular INR monitoring in months 1–6 (median number of INRs 11, IQR 7–16), 16 639 (41.0%) met the definition of good control (TTR > 65%); good control continued to be exhibited by 8177 (57.1% of those who remained on warfarin) during months 7–12 and 6804 (56.8% of continuing warfarin users) in months 13–18. Good control in the first 6 months predicted good control over the subsequent year: adjusted OR (aOR) 4.0(95%CI 3.8 to 4.2), c index 0.685(95%CI 0.679 to 0.691) for months 7–12 and aOR 3.2(95%CI 3.1 to 3.3), c index 0.665(95%CI 0.659 to 0.671) for months 13–18. CONCLUSIONS: Nearly one-third of warfarin-treated patients had insufficient INR monitoring—this could influence the initial choice of anticoagulant and identifies a target for future quality improvement efforts. Of those warfarin-treated patients who went for regular INR monitoring, 41% exhibited levels of control similar to that in randomised trials and this deteriorated by half over time. However, in patients who have already exhibited adherence with regular monitoring and good TTR, warfarin may still be a reliable anticoagulation option.