To the Editor Dr Carrero and colleagues¹ evaluated the influence of chronic kidney disease (CKD) on the efficacy and safety of warfarin therapy in a population with atrial fibrillation and recent myocardial infarction. Previous studies have found that warfarin, when added to aspirin or dual antiplatelet therapy, is associated with increased risk of major bleeding.^2,3 We thus find it surprising that, in this registry-based Swedish investigation, no excess bleeding risk was observed in warfarin users across stages of CKD. Accordingly, the characteristics of the study population deserve careful examination.

Only 21.8% of patients with atrial fibrillation were prescribed warfarin, despite more than 75% of patients having a CHADS₂ score of 2 or higher, perhaps reflecting stringent selection of patients with low overall bleeding risk. The reasons for withholding warfarin were not detailed. Moreover, Scandinavian cohorts typically have a higher prevalence of time in therapeutic range while taking warfarin, perhaps further curtailing bleeding risk, compared with the general population.