Indexing for body surface area when assessing kidney function.

The glomerular filtration rate (GFR) indexed to a standardized body surface area (BSA) of 1.73 m² is the cornerstone metric for kidney function assessment, allowing comparisons across individuals with diverse body sizes. This review outlines the historical development of BSA estimation methods and the rationale behind indexing GFR. While BSA-indexed GFR is useful for population-level comparisons and chronic kidney disease staging, non-indexed GFR reflects renal clearance and should be preferred for specific purposes, such as drug dosing. In addition, it is discussed whether to use indexed or non-indexed GFR when deciding to accept a kidney for transplantation for a patient with kidney failure, or when assessing GFR over time in individuals with severe weight change. Alternative indexing methods based on lean body mass, total body water, or extracellular fluid volume show theoretical potential but lack established clinical benefit. Future research may explore novel indexing methods that balance physiological accuracy and clinical practicality. For now, BSA-indexed GFR remains the clinical standard to assess and monitor kidney function.