In Reply to ‘The Importance of Considering Metabolism When Indexing the GFR’

Abstract

We appreciate Dr Friedman's1Friedman A.N. The importance of considering metabolism when indexing the GFR.Am J Kidney Dis. 2010; 56: 1218Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar interest in our editorial entitled “Obesity, Glomerular Hyperfiltration, and the Surface Area Correction,”2Levey A.S. Kramer H. Obesity, glomerular hyperfiltration, and the surface area correction.Am J Kidney Dis. 2010; 56: 255-258Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar which accompanied the article “Marked Association Between Obesity and Glomerular Hyperfiltration: A Cross-sectional Study in an African Population.”3Wuerzner G. Pruijm M. Maillard M. et al.Marked association between obesity and glomerular hyperfiltration: a cross-sectional study in an African population.Am J Kidney Dis. 2010; 56: 303-312Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar In this editorial, we discussed several issues regarding obesity and kidney disease, including indexing of glomerular filtration rate (GFR) by body surface area (BSA). Indexing GFR for BSA historically has been used to normalize GFR for differences in body size because metabolic rate, which generally increases with increasing body size, is linked to GFR, as shown by power law equations.4Singer M.A. Of mice and men and elephants: metabolic rate sets glomerular filtration rate.Am J Kidney Dis. 2001; 37: 164-178Abstract Full Text PDF PubMed Scopus (130) Google Scholar Dr Friedman points out that metabolic rate is not necessarily proportional to BSA, which is calculated separately for men and women using height and weight. He suggests that indexing GFR by BSA is not appropriate because BSA ignores differences in fat-free mass. We agree that as a person gains weight, the accompanying increase in lean body mass is not necessarily proportional to the total weight gain. However, indexing GFR by BSA has become part of routine clinical practice and is used in guidelines for chronic kidney disease risk stratification.5National Kidney FoundationK/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: evaluation, classification and stratification.Am J Kidney Dis. 2002; 39: S46-S64Google Scholar Before exploring a new measure to index GFR, research should first determine whether current reference values for GFR indexed by BSA in obese individuals leads to appropriate or inappropriate care and risk stratification for chronic kidney disease outcomes. Financial Disclosure: The authors declare that they have no relevant financial interests The Importance of Considering Metabolism When Indexing the GFRAmerican Journal of Kidney DiseasesVol. 56Issue 6PreviewI would like to expand upon Levey and Kramer's1 thoughtful discussion about the appropriateness of indexing glomerular filtration rate (GFR) by body surface area (BSA) to standardize measurements by different body sizes. The BSA adjustment is premised upon proportional relationships between renal physiologic parameters, such as GFR, kidney size, and body size, relationships that can be summarized in an allometric power law equation that takes into account body weight.2 The presumed relevance of body weight is its linkage to metabolic load; that is, a large body is expected to generate more waste than a smaller one, with the kidney modifying its excretory and other capabilities accordingly to meet the body's demands. Full-Text PDF