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.

The Impact of Renal Function on Outcomes of Bariatric Surgery

“…GFR corrected for BSA in obese individuals may lead to underestimation of GFR, and using a nonindexed BSA may be preferable in the obese population. 26,27,32,33 Although differences in BSA may exist, it is unclear whether the accuracy of the equations is improved by correcting for BSA. 25 In our analysis we adjusted for BSA and found no change in the conclusions (data not shown).…”

Extracellular fluid volume and glomerular filtration rate

“…Part of this apparent expansion, however, might be explained by the tendency for diabetic patients to have a larger BSA, scaling to which has the misleading tendency of raising ECV/BSA and GFR/BSA as a result of the nonlinear relationship of both ECV and GFR with BSA. Thus, Peters and Glass [17] recently described the 'BSA effect' on GFR scaling and contrasted it with the 'obesity effect'; therefore, it is noteworthy that the patient group had an overall mean BSA higher than the normal volunteers, which would tend to give higher values of ECV/BSA in the patient group.…”

“…In a recent evaluation of the suitability of BSA to scale GFR in obese compared with nonobese individuals, for instance, Peters and Glass [17] argued in favour of the use of ECV as a surrogate for GFR because they perceived ECV as less variable than GFR in a clinical population of patients. This study supports this perception.…”

Measurement of Glomerular Filtration Rate in Obese Patients: Pitfalls and Potential Consequences on Drug Therapy

“…1). Other ways to index for GFR have been proposed, which include extracellular volume [15], height [14] and lean body mass [16,17]. Interestingly, the latter has been proposed as a good predictor of drug dosage in obese patients [18].…”