Most patients taking warfarin do not maintain stable INR valuesBMJ 2016; 354 doi: https://doi.org/10.1136/bmj.i4402 (Published 10 August 2016) Cite this as: BMJ 2016;354:i4402
Most patients taking warfarin who have stable international normalised ratio (INR) values do not maintain them over time, a study published in JAMA has found. The finding challenges the belief that patients with good outcomes from warfarin should keep taking it rather than switching to non-vitamin K oral anticoagulants.1
Warfarin substantially decreases stroke risk in patients with atrial fibrillation, but it has a narrow therapeutic window (INR values 2.0 to 3.0) and is associated with multiple drug and food interactions. Non-vitamin K oral anticoagulants do not need drug monitoring and have similar or improved safety and efficacy relative to warfarin, but they are more expensive. It is controversial whether patients who were previously stable taking warfarin should be switched to non-vitamin K oral anticoagulants.
The study included 3749 patients with atrial fibrillation who were taking warfarin at 176 clinics in the United States. The average age was 75, and 43% were women. Only 26% of the patients had 80% or more of their INR values in the 2.0-3.0 range during the first six months. Of patients with stable INRs during the first six months, only 34% continued to have stable results the next year.
Whether patients were stable during the baseline period was found to be of limited value in predicting whether they would remain stable over the subsequent year. Among the patients with 80% or more INRs in the target range at the start of the study, 36% had at least one extreme INR value in the following year.
A limitation to the study was the assumption that the target INR range was 2.0-3.0 when doctors could have targeted lower INR values due to bleeding or higher values for mechanical valves.
However, the authors wrote, “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.
“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.”