INR Stability Hard to Predict with Long-Term Warfarin Use
A study published in JAMA found that warfarin use to prevent strokes in atrial fibrillation may not adequately manage blood clotting over time, despite patients having been stable on the drug.
Researchers from Duke University Medical Center evaluated whether patients receiving warfarin who have stable INR values remain stable over time. Though warfarin decreases stroke risk among patients with atrial fibrillation, it has a narrow therapeutic window (INR 2.0–3.0) in addition to multiple drug and food interactions. In contrast, non-vitamin K oral anticoagulants are more expensive but do not require drug monitoring and have comparable or improved safety and efficacy relative to warfarin.
The 18-month study included data from a prospective registry of patients with atrial fibrillation who were enrolled in clinics from June 2010 through August 2011 with a 3-year follow-up. Stability was defined as ≥80% INRs in therapeutic range (2.0–3.0). Of the total 10,132 patients in the registry, 6,383 were excluded because they were not taking warfarin or had insufficient INR values.
Of the 3,749 patients taking warfarin, 26% (n=968) had ≥80% INR values in the 2.0–3.0 range during the first 6 months; 34% of these patients remained stable over the subsequent year. Stability at baseline was a limiting predictor of stability for the year. Among those with stable INR ranges at baseline, 36% had ≥1 well-out-of-range INR in the subsequent year, further showing the limited predictability of stability. This finding was significant because the risk of clotting and stroke increases most with INR values <1.5 and risks of brain bleeding significantly increases with INR values >4.
"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," study authors concluded. Non-vitamin K oral anticoagulant therapy may not be appropriate for every patient, however the study's findings show all eligible patients and even those who have performed well with warfarin in the past, should discuss with their clinician the benefits and risks of a therapy change.
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