Twelve-month outcomes and predictors of very stable INR control in prevalent warfarin users.

Background: For patients on warfarin therapy an international normalized ratio (INR) recall interval not exceeding 4 weeks has traditionally been recommended. For patients whose INR values are nearly always therapeutic, less frequent INR monitoring may be feasible.

Objective: To identify patients with stable INRs (INR values exclusively within the INR range) and comparator patients (at least one INR outside the INR range), compare occurrences of thromboembolism, bleeding and death between groups, and identify independent predictors of stable INR control.

Methods: The study was a retrospective, longitudinal cohort study using data extracted from electronic databases. Patient characteristics and risk factors were entered into multivariate logistic regression models to identify variables that independently predict stable INR status.

Results: There were 533 stable and 2555 comparator patients. Bleeding and thromboembolic complications were significantly lower in stable vs. comparator patients (2.1% vs. 4.1% and 0.2% vs. 1.3%, respectively; P < 0.05). Independent predictors of stable INR control were age >70 years, male gender and the absence of heart failure. Stable patients were significantly less likely to have target INR > or =3.0 or chronic diseases.

Conclusion: A group of patients with exclusively therapeutic INR values over 12 months is identifiable. In general, these patients are older, have a target INR <3.0, and do not have heart failure and/or other chronic diseases. Our findings suggest that many patients whose INR values remain within the therapeutic range over time could be safely treated with INR recall intervals >4 weeks.