Evaluation of INR Monitoring Frequency and Time in Therapeutic Range

Abstract

Background: Although anticoagulant therapy has been used for more than 50 years, the optimal interval between international normalized ratio (INR) measurements in patients who demonstrate a stable dose response remains unsettled. Based on the results of observational studies, the American College of Chest Physicians currently recommends that patients receiving a stable dose of warfarin undergo INR testing at least once every 4 weeks.1 Objective: To determine whether an increased frequency of INR monitoring impacts time in therapeutic range (TTR) among patients receiving maintenance warfarin therapy managed by an anticoagulation clinic. Methods: A total of 2,222 INR results belonging to more than 700 adults who were followed by the anticoagulation service at the University of Pittsburgh Medical Center and who were receiving maintenance warfarin therapy between April 1, 2006, and September 30, 2006, were retrospectively reviewed. INR measurements were categorized as having been determined fewer than 21 days or 21 or more days after the previous measurement, and the percentage of INRs within the therapeutic range was calculated for each interval. Results: During the evaluation period, INRs measured within 21 days of the preceding test were within the therapeutic range 75.1% of the time, whereas INRs obtained at least 21 days after the antecedent INR test were within the therapeutic range 79.9% of the time (p < 0.001). Additionally, the proportion of INRs below 1.5 or above 5.0 was not significantly different between the 2 groups, representing only 0.04% of all results at either interval (p > 0.05). Conclusions: Our evaluation suggests that more frequent INR monitoring among patients receiving maintenance warfarin does not impact TTR to a clinically meaningful extent. Moreover, these findings support the reevaluation of the anticoagulation service management algorithm to reduce the frequency of INR measurements among patients within the maintenance phase of therapy.