Percent Time in Range with Warfarin as a Performance Measure: How Long a Sampling Frame Is Needed?

Abstract

Warfarin is received by millions of patients in the United States and elsewhere and will remain the most commonly used anticoagulant for the foreseeable future. Percent time in therapeutic range (TTR) with warfarin is increasingly used as a performance measure. However, stakeholders have expressed concern that TTR lags behind changes in performance. Work in a larger study focused on the impact of shortening the conventional measurement period for TTR. Some 124 sites within the Department of Veterans Affairs (VA) were examined during a seven-year period (fiscal years [FYs] 2008-2014 (April 1, 2007-September 30, 2014). The duration of time segments (2, 3, 4, 6 months) used to calculate TTR were varied, and these four durations were compared in terms of the number of patients retained per site, mean and median site TTR, and site performance rankings. Data were obtained on 295,237 unique patients who received anticoagulation. As the calculation window shortened, patients with better control (that is, higher TTR) were selectively excluded from the measurement because their laboratory values were more widely spaced. Site mean TTR was highest when the most patients were included (6 months: 950 patients; TTR 65.2%) and lowest when the fewest patients were included (2 months: 567 patients; TTR 60.0%). However, the 3-, 4-, and 6-month segments achieved similar results, each of which included more than 800 patients per site, with mean TTR across a narrow range (64.9%-65.2%). Site rankings were less highly correlated between the 2-month period and longer periods (r = 0.7- 0.8) but were otherwise 0.95 or higher, with a nearly perfect correlation (0.985) between the 4- and 6-month periods. When TTR is used to measure site-level performance, comparable results can be achieved using a 4- or a 6-month measurement period. On the basis of these results, the use of a 4-month period for future measurement efforts is recommended.