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.
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).
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).
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.