Glomerular filtration rate (GFR) is traditionally indexed to body surface area (BSA) to allow comparison among individuals of different body size, and normative values for GFR and the definitions of acute and chronic kidney disease are based on BSA-indexed GFR.1,2 Modern GFR estimating equations were developed using measured GFR (mGFR) indexed for each participant’s BSA and report estimated GFR (eGFR) indexed to 1.73 m2.3,4 However, some clinical settings may favor using nonindexed instead of indexed GFR; for example, for dosing of drugs eliminated by the kidney when BSA is very different from the index value of 1.73 m2 (eg, with obesity or anorexia nervosa), or for evaluation of change in GFR after a large change in BSA (eg, after bariatric surgery).