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Pokorney SD, Simon DN, Thomas L, et al. Stability of International Normalized Ratios in Patients Taking Long-term Warfarin Therapy. JAMA. 2016;316(6):661–663. doi:10.1001/jama.2016.9356
Warfarin substantially decreases stroke risk among patients with atrial fibrillation yet has a narrow therapeutic window (international normalized ratio [INR] values of 2.0-3.0) and is associated with multiple drug and food interactions.1 Non–vitamin K oral anticoagulants do not require drug monitoring and have similar or improved safety and efficacy relative to warfarin but are more costly.2 Whether patients previously stable on warfarin should be switched to non–vitamin K oral anticoagulants remains controversial but may be informed by determining whether patients receiving warfarin who have stable INR values remain stable over time.
Written informed consent was obtained for registry enrollment. The Duke institutional review board approved this study. Data were obtained from the Outcomes Registry for Better Informed Treatment of Atrial Fibrillation, a prospective registry of patients with atrial fibrillation from 176 clinics who were enrolled June 2010 through August 2011 and followed up for 3 years through November 2014.3 Patients receiving warfarin at baseline with 3 or more INR values in the first 6 months and 6 or more in the subsequent year were included. Stability was defined as 80% or more INRs in therapeutic range (2.0-3.0). An R2 value assessed stability in the first 6 months as a predictor of stability in the subsequent year (both continuous). A C index determined whether stable INR during the baseline 6 months discriminated stable INR in the subsequent year.
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