The analyses by Jun et al1 and Shen et al2 add to the growing list of observational studies suggesting a net clinical benefit for warfarin in patients with chronic kidney disease (CKD) with atrial fibrillation. The debate about warfarin safety in CKD is ongoing, with as many proponents as opponents. We want to emphasize the possible confounding effect of the quality and intensity of anticoagulation control using international normalized ratios (INRs) and time in therapeutic range (TTR). This information is usually lacking in administrative records, but it is of fundamental importance when assessing warfarin safety and effectiveness in clinical practice.