Re-Examining the Recommended Follow-Up Interval after an In-Range INR Value: Results from the Veterans AffaiRs Study to Improve Anticoagulation (VARIA)

Adam J. Rose, Bedford VA Medical Center
Al Ozonoff, Boston Children's Hospital
Dan R. Berlowitz, Boston University School of Medicine
Arlene S. Ash, University of Massachusetts Medical School
Joel I. Reisman, Bedford VA Medical Center
Elaine M. Hylek, Boston University School of Medicine

Abstract

BACKGROUND: Patients receiving oral anticoagulation should be tested often enough to optimize control, but excessive testing increases burden and cost. We examined the relationship between the follow-up interval after an in-range (2.0-3.0) INR and anticoagulation control.

METHODS: We studied 104,451 patients receiving anticoagulation from 100 anticoagulation clinics in the Veterans Health Administration (VA). Most patients (98,877) recorded at least one in-range INR followed by another INR within 56 days. For each such patient, we selected the last in-range INR and characterized the interval between this "index value" and the next INR. The independent variable was site mean follow-up interval after an in-range INR. The dependent variable was site mean risk-adjusted percent time in therapeutic range (TTR).

RESULTS: Site mean interval varied from 25-38 days. As the mean site follow-up interval became longer, risk-adjusted TTR was worse (-0.51% per day, p = 0.004). This relationship persisted when the index value was the first consecutive in-range INR (-0.63%, p < 0.001), or the second (-0.58%, p < 0.001), but not the third or greater (-0.12%, p = 0.46).

CONCLUSIONS: Sites varied widely regarding follow-up intervals after in-range INR (25-38 days). Shorter intervals were generally associated with better anticoagulation control, but after a third consecutive in-range value, this relationship was greatly attenuated and no longer statistically significant. Our results suggest that a maximum interval of 28 days after the first or second in-range value and consideration of a longer interval after the third or greater consecutive in-range value may be appropriate.