Performance of stroke risk scores in older people with atrial fibrillation not taking warfarin: comparative cohort study from BAFTA trial

Abstract

Objective: To compare the predictive power of the main existing and recently proposed schemes for stratification of risk of stroke in older patients with atrial fibrillation.

Design: Comparative cohort study of eight risk stratification scores.

Setting: Trial of thromboprophylaxis in stroke, the Birmingham Atrial Fibrillation in the Aged (BAFTA) trial.

Participants: 665 patients aged 75 or over with atrial fibrillation based in the community who were randomised to the BAFTA trial and were not taking warfarin throughout or for part of the study period.

Main outcome measures: Events rates of stroke and thromboembolism.

Results: 54 (8%) patients had an ischaemic stroke, four (0.6%) had a systemic embolism, and 13 (2%) had a transient ischaemic attack. The distribution of patients classified into the three risk categories (low, moderate, high) was similar across three of the risk stratification scores (revised CHADS(2), NICE, ACC/AHA/ESC), with most patients categorised as high risk (65-69%, n = 460-457) and the remaining classified as moderate risk. The original CHADS(2) (Congestive heart failure, Hypertension, Age ≥ 75 years, Diabetes, previous Stroke) score identified the lowest number as high risk (27%, n = 180). The incremental risk scores of CHADS(2), Rietbrock modified CHADS(2), and CHA(2)DS(2)-VASc (CHA(2)DS(2)-Vascular disease, Age 65-74 years, Sex) failed to show an increase in risk at the upper range of scores. The predictive accuracy was similar across the tested schemes with C statistic ranging from 0.55 (original CHADS(2)) to 0.62 (Rietbrock modified CHADS(2)), with all except the original CHADS(2) predicting better than chance. Bootstrapped paired comparisons provided no evidence of significant differences between the discriminatory ability of the schemes.

Conclusions: Based on this single trial population, current risk stratification schemes in older people with atrial fibrillation have only limited ability to predict the risk of stroke. Given the systematic undertreatment of older people with anticoagulation, and the relative safety of warfarin versus aspirin in those aged over 70, there could be a pragmatic rationale for classifying all patients over 75 as "high risk" until better tools are available.

Risk of ischaemic stroke (point estimates with 95% confidence interval) by individual CHADS₂ score