Abstract 5795: Impact of the Modified Henry Ford Warfarin Maintenance Dosing Algorithm on Quality of Anticoagulation at a Specialist Anticoagulation Clinic

Purpose: The efficacy of warfarin depends on the time that the international normalized ratio (INR) is in the target therapeutic range. The time-in-therapeutic range (TTR) is a measure of anticoagulation quality, and should be maximized. The objective of our before-after study was to determine whether routine use of a simple manual warfarin dosing algorithm compared with ‘expertise-based’ dosing can improve the TTR for warfarin. Methods: The study was performed at a single anticoagulation clinic in Hamilton, Canada. In the ‘before’ phase (August, 2006 until September, 2007) we retrospectively calculated in patients on warfarin: agreement between warfarin dosing and the modified Henry Ford dosing algorithm and TTR, using Rosendaal’s linear interpolation method. In this period, warfarin was managed by experienced anticoagulation clinic physicians. In the ‘after’ phase (July until December, 2008) we prospectively calculated the same parameters. Differences between ‘before’ and ‘after’ were tested with the independent t-test for continuous and chi-square for dichotomous variables. Results: We included 873 patients in the ‘before’ phase and 1,088 patients in the ‘after’ phase. Before introduction of the algorithm, 71% of warfarin dose adjustments were consistent with the algorithm in patients targeting an INR of 2–3 and 56% for those targeting INR 2.5–3.5, whereas after algorithm introduction agreement increased to 90% and 81%, respectively. Introduction of the dosing algorithm significantly increased the TTR in patients targeting an INR of 2–3 from 67.2% to 73.2% (p<0.001) and in those targeting an INR of 2.5–3.5 from 49.8% to 63.8% (p<0.001). This improvement of TTR was due to a decreased proportion of subtherapeutic INRs, from 18 to 13% (p<0.01) with target INR 2–3 and from 41 to 25% (p<0.01) with target INR 2.5–3.5, while the proportion of supratherapeutic INRs did not significantly change. Conclusion: Introduction of the modified Henry Ford manual warfarin maintenance dosing algorithm in place of expertise-based dosing significantly improved the mean TTR in our tertiary care anticoagulation clinic. This widely applicable algorithm could be an inexpensive evidence-based method to improve warfarin control and thereby patient outcomes.