Durham, NC—Deciding whether to switch patients with stable internal normalized ratio (INR) values on warfarin to newer anticoagulants can be a difficult decision.
A study published as a research letter in JAMA, however, provides some important new information.
Researchers from Duke University Medical Center and colleagues examined whether INR levels remain stable over time in patients receiving warfarin, which substantially decreases stroke risk among patients with atrial fibrillation but has a narrow therapeutic window, i.e., INR values of 2.0 to 3.0.
“Whether patients previously stable on warfarin should be switched to non-vitamin K oral anticoagulants remains controversial but may be informed by determining whether patients receiving warfarin who have stable INR values remain stable over time,” study authors emphasize.
Warfarin is associated with multiple drug and food interactions, compared to nonvitamin K oral anticoagulants. The newer therapies don’t require drug monitoring and have similar or improved safety and efficacy relative to warfarin but cost more, according to background information in the article.
For the study, researchers obtained data from a prospective registry of patients with atrial fibrillation from 176 clinics, including 3,749 receiving warfarin at study entry with three or more INR values in the first 6 months and six or more in the subsequent year. Participants, average age of 75, were enrolled June 2010 through August 2011 and followed for 3 years through November 2014. Stability was defined as 80% or more INRs in therapeutic range, and 26% of the patients met that goal in the first 6 months.
Results indicate that, of patients with stable INRs during the first 6 months, 34% remained stable over the subsequent year, indicating that stability during the baseline period had limited predictive ability of stability over the subsequent year.
In fact, the study points out, among patients with 80% or more INRs in range at baseline, 36% had one or more well-out-of-range INR in the following year.
“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,” study authors write. “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,”
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