In reply
Drs Makin, Conway, and Lip have raised a concern that we may have overestimated the risk of cardiogenic embolism in patients with AF.1
The main strengths of our study were its large size; the uniformly organized health care system with primary catchment areas, which allowed a population-based design, thus avoiding the usual bias in hospital-based studies introduced by differential patient recruitment; and the completeness of follow-up. However, our study had several limitations. We included only subjects who had a hospital discharge diagnosis of AF, and we lacked clinical details such as smoking history and claudication. Nondifferential misclassification of AF, comorbid conditions, and outcome (systemic thromboembolism) may have occurred, and may have tended to bias toward the null the estimate of the relative risk of systemic thromboembolism. Thus, our estimates of the risk of a systemic thromboembolic event associated with AF are conservative.1