Warfarin treatment in patients with atrial fibrillation: observing outcomes associated with varying levels of INR control.

INTRODUCTION: We aimed to determine the level of INR control associated with reduced stroke and mortality.

MATERIAL AND METHODS: The study used a retrospective cohort design using linked inpatient, haematology and mortality data from Cardiff and the Vale of Glamorgan, UK. Anonymised patients admitted with a diagnosis of non-valvular atrial fibrillation (NVAF) were defined as warfarin or non-warfarin treated by number of repeated International Normalised Ratio (INR) tests. Warfarin treated patients (>5 INR tests) categorised as at moderate or high risk of stroke (CHADS2 score > or = 2) with varying levels of INR control were compared to those who did not receive warfarin treatment using Cox proportional hazards models controlling for age, sex and CHADS2 score. Outcome measures were time to stroke and mortality.

RESULTS: 6,108 patients with NVAF were identified. 2,235 (36.6%) of these patients had five or more INR readings and of these 486 (21.7%) had CHADS2 score > or = 2. There was significant improvement in time to stroke event in those patients with INR control of greater than 70% of time in therapeutic range (2.0 to 3.0) compared with the non-warfarin treatment group. Overall survival was significantly improved for all warfarin treated groups with INR control of greater than 40% of time in range.

CONCLUSIONS: Patients with INR control of above 70% of time in range had a significantly reduced risk of stroke. Patient suitability for warfarin treatment should be continuously assessed based on their ability to maintain a consistently therapeutic INR.