Background- Improved control of oral anticoagulation reduces adverse events. A program of quality measurement is needed for oral anticoagulation. The interval until the next test after an out-of-range International Normalized Ratio (INR) value (the "follow-up interval") could serve as a process of care measure.

Methods and Results- We studied 104,451 patients cared for by 100 anticoagulation clinics in the Veterans Health Administration (VA). For each site, we computed the average follow-up interval after low (≤1.5) or or high (≥4.0) INR. Our outcome was each site's average anticoagulation control, measured by percent time in therapeutic range (TTR); 59 837 patients (57%) contributed to the low INR analysis, 37 697 (36%) contributed to the high INR analysis, and all patients contributed to the dependent variable (mean site TTR). After a low INR, site mean follow-up interval ranged from 10 to 24 days. Longer follow-up intervals were associated with worse site-level control (1.04% lower for each additional day, P<0.001). After a high INR, site mean follow-up interval ranged from 6 to 18 days, with longer follow-up intervals associated with worse site-level control (1.12% lower for each additional day, P<0.001). These relationships were somewhat attenuated but still highly statistically significant when the proportion of INR values in-range was used as the dependent variable rather than TTR.

Conclusions- Prompt repeat testing after out-of-range INR values is associated with better anticoagulation control at the site level and could be an important part of a quality improvement effort for oral anticoagulation.