Effect of a simple two-step warfarin dosing algorithm on anticoagulant control as measured by time in therapeutic range: a pilot study

Background: The efficacy and safety of vitamin K antagonists for the prevention of thromboembolism are dependent on the time for which the International Normalized Ratio (INR) is in the therapeutic range. The objective of our study was to determine the effect of introducing a simple two-step dosing algorithm, as compared with dosing by anticoagulation clinic staffs on the basis of their experience, on time in therapeutic range (TTR) of warfarin therapy. Methods: We compared TTRs of all clinic patients before and after the introduction of a simple two-step dosing algorithm at a single anticoagulation clinic in Canada, between 1 August 2006 and 24 December 2008. TTR was calculated using the linear interpolation method of Rosendaal. Results: We included 873 patients in the 'before' phase and 1088 patients in the 'after' phase. Introduction of the dosing algorithm significantly increased TTR of patients with a therapeutic INR range of 2-3 from 67.2% to 73.2% (P < 0.001), and that of patients with a therapeutic INR range of 2.5-3.5 from 49.8% to 63.8% (P < 0.001). Conclusions: The introduction of a simple two-step warfarin-dosing algorithm in place of dosing by experienced anticoagulation clinic staff significantly improved mean TTR for patients in a tertiary-care anticoagulation clinic. This inexpensive and widely applicable algorithm has the potential to improve warfarin control.