Use of Body Surface Area for Assessing Extracellular Fluid Volume and Glomerular Filtration Rate in Obesity

Abstract

Objective: To examine body surface area (BSA) for scaling extracellular fluid volume (ECV) in obesity. ECV varies less than glomerular filtration rate (GFR) in a clinical population and was therefore used as a surrogate for GFR on the grounds that if BSA is unsuitable for scaling GFR, it will also be unsuitable for ECV. Methods: GFR was measured in 917 patients using ⁵¹Cr-EDTA. GFR scaled to ECV was measured exclusively from the slope rate constant. ECV was calculated as GFR divided by GFR/ECV. Results: BSA correlated strongly with body mass index (BMI). ECV correlated strongly with BSA but the intercept was significantly lower than zero, indicating a disproportionate relation. ECV/BSA correlated with BSA but not with BMI. ECV in obese subjects was significantly less than in non-obese subjects individually matched for BSA. ECV/BSA was similar between obese and lean subjects matched for GFR/ECV and height. Conclusions: For subjects of similar BSA, a high BMI decreases ECV (the ‘obesity effect’). Subjects with high BMI generally have high BSA, which tends to increase ECV/BSA because of the disproportionate relation between ECV and BSA (the ‘BSA effect’). These opposing effects serendipitously and erroneously create the impression that BSA is suitable for scaling ECV (and by implication, GFR) in obesity.