Stable INR values short-lived for majority of warfarin users
medwireNews: Only one-third of patients who achieve a stable international normalised ratio (INR) during 6 months of warfarin therapy will maintain that stability during the subsequent year’s treatment, US researchers report.
“A common belief has been that patients with stable INRs while taking warfarin would continue to be stable and derive less benefit from switching to non-vitamin K oral anticoagulants”, say Eric Peterson (Duke University Medical Center, Durham, North Carolina) and colleagues.
“This analysis suggests warfarin stability is difficult to predict and challenges the notion that patients who have done well taking warfarin should maintain taking warfarin”, they add.
The study included 3749 patients with atrial fibrillation who were receiving warfarin at study entry with three or more INR values in the first 6 months and six or more in the subsequent year. All patients were enrolled in the prospective Outcomes Registry for Better Informed Treatment of Atrial Fibrillation and followed up for 3 years.
During the first 6 months, 968 (26%) patients had a stable INR, defined as at least 80% of INR values within the therapeutic range of 2.0 to 3.0.
Of these, just 34% remained stable over the subsequent year and 36% had at least one well-out-of-range INR (<1.5 or >4.0) during this time.
Statistical analysis showed that prior stability explained just 11% of variation in subsequent stability. Furthermore, stability during the initial 6-month period had limited ability to predict stability over the subsequent year, with a C-statistic of 0.61.
The researchers also identified 376 (10%) patients with 100% of INR values in range during the first 6 months. Stability in the subsequent year was only slightly better in these patients, with 37% remaining stable and 33% having one or more well-out-of-range INR.
Analyses of time in therapeutic range (TTR) gave similar results: 32% of patients had a TTR of 80% or more in the first 6 months. Among these patients, 42% achieved the same target during the subsequent 12 months and 38% reported at least one INR well out of range.
The study findings are published in a letter to JAMA.
By Laura Cowen
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