Lyu et al1 evaluated discrepancies between different glomerular filtration rate (GFR) estimating equations among 30,261 patients with atrial fibrillation. In patients with low body mass index (BMI), they observed that GFR was lowest according to estimated creatinine clearance (eCrCl), highest according to estimated GFR (eGFR) indexed to body surface area, and intermediate according to nonindexed eGFR. In patients with high BMI, they observed the opposite trend. However, this conclusion is a mathematical consequence of the equations themselves and does not require any analysis of patient data.2