Results of a large study suggest most patients taking warfarin long-term do not maintain stable international normalized ratio (INR) values.
Researchers analyzed data from more than 3700 patients and found that 74% did not have stable INRs during the first 6 months of analysis.
Of the patients who did have stable INRs during this period, only 34% maintained stable INRs over the subsequent year.
Sean D. Pokorney, MD, of Duke University Medical Center in Durham, North Carolina, and his colleagues reported these findings in JAMA.
The researchers analyzed data from a prospective registry of patients with atrial fibrillation treated at 176 clinics. The patients were enrolled from June 2010 through August 2011 and followed for 3 years through November 2014.
Patients receiving warfarin at study entry with 3 or more INR values in the first 6 months and 6 or more in the subsequent year were included.
Of 10,132 registry patients, 6383 were not taking warfarin or had insufficient INR values and were excluded.
So there were 3749 eligible patients taking warfarin. Their average age was 75. Forty-three percent of patients were female, and 91% self-identified as white.
The patients’ median time from their first warfarin prescription to baseline was 3.9 years (range, 1.5-7.5 years). Thirty-seven percent of patients were also taking aspirin, and 5% were taking clopidogrel.
Results
INR stability was defined as 80% or more INRs in the therapeutic range (2.0-3.0).
Twenty-six percent of the patients met this definition during the first 6 months, and 34% of these patients maintained stable INRs over the subsequent year.
Ten percent of all patients had 100% of their INR values in the therapeutic range during the first 6 months. Of these patients, 37% met the definition of stability over the subsequent year.
Of the patients who had 80% or more of their INR values in the therapeutic range at baseline, 36% had 1 or more well-out-of-range INRs in the following year.
Of the patients with 100% of their baseline INR values in the therapeutic range, 33% had 1 or more well-out-of-range INRs in the subsequent year.
The researchers said these results suggest warfarin stability is difficult to predict, and this study challenges the notion that patients who have done well taking warfarin should continue taking warfarin.
