Reexamining the recommended follow-up interval after obtaining an in-range international normalized ratio value: results from the Veterans Affairs study to improve anticoagulation

Abstract

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

Methods: We studied 104,451 patients who were receiving anticoagulation therapy from 100 anticoagulation clinics in the US Veterans Health Administration. Most patients (98,877) had 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 the site mean follow-up interval after obtaining an in-range INR. The dependent variable was the site mean risk-adjusted percentage of time in the therapeutic range (TTR).

Results: The site mean follow-up interval varied from 25 to 38 days. As the site mean follow-up interval became longer, the risk-adjusted TTR was worse (-0.51% per day, P = .004). This relationship persisted when the index value was the first consecutive in-range INR (-0.63%, P < .001) or the second (-0.58%, P < .001), but not the third or greater (-0.12%, P = .46).

Conclusions: Sites varied widely regarding follow-up intervals after obtaining an in-range INR (25-38 days). Shorter intervals were generally associated with better anticoagulation control, but after obtaining 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 obtaining the first or second in-range value and consideration of a longer interval after obtaining the third or greater consecutive in-range value may be appropriate.