Stroke and thromboembolic event rates in atrial fibrillation according to different guideline treatment thresholds: A nationwide cohort study

Contemporary guidelines suggest anticoagulant treatment decisions in atrial fibrillation (AF) patients to be based on risk stratification for stroke. However, guidelines do not agree on the threshold for treatment initiation. We explored the variation in thromboembolic event rates in a non-anticoagulated AF population, according to different guideline threshold and methodological approaches. AF patients between 1998 and 2014 free from anticoagulant treatment were identified. Event rates for ischemic stroke and ischemic stroke/systemic embolism were explored. The overall ischemic stroke rate was 3.20 per 100 person-years ('formal rate assessment'). For patients with a CHA(2)DS(2)-VASc score of 1 the ischemic stroke rate was 0.97 when using a 'formal rate assessment', 0.62 when using a 'conditioning on the future' approach, and 0.93 when using a 'censoring approach'. Rates for thromboembolism for the 'European treatment threshold' (CHA(2)DS(2)-VASc score of 1, males only) ranged 1.17 to 1.53. Rates for the 'U.S. treatment threshold' (CHA(2)DS(2)-VASc of 2) ranged from 1.95 to 2.33. Thromboembolic event rates differed markedly in non-anticoagulated AF patients according to the conflicting European and U.S. guideline treatment thresholds. Second, the choice of methodological approach has implications, thus we recommend using the censoring approach for event rate estimation among AF patients not on treatment.