FALLACY OF INDEXING RENAL AND SYSTEMIC HEMODYNAMIC MEASUREMENTS FOR BODY-SURFACE AREA

Renal plasma flow, glomerular filtration rate, and cardiac output are traditionally indexed for body surface area by expressing these traits as per-surface-area ratios. Indexing is intended to remove interindiv idual variation attributable to differences in body size. Regression i s an alternative method commonly used to adjust other biological trait s for the effects of a covariate, such as body surface area. The purpo se of this study was to compare the indexing and regression methods of adjusting renal plasma flow, glomerular filtration rate, and cardiac output for interindividual differences in body surface area. We estima ted renal plasma flow by the clearance of p-aminohippurate, glomerular filtration rate by clearance of inulin, and cardiac output by thoraci c electrical impedance in a sample of 78 unrelated females and 78 unre lated males (ages 20-49.9 yr) from the general population of Rochester , MN. The indexing method created negative dependencies of renal plasm a flow and cardiac output on body surface area and failed to eliminate the positive dependency of glomerular filtration rate on body surface area. Moreover, indexing obscured differences in mean renal plasma fl ow between females and males and created differences in mean cardiac o utput between the genders, In contrast, the regression method consiste ntly eliminated dependencies of each trait on body surface area and di d not lead to inappropriate inferences about mean differences in these traits between females and males. We conclude that the indexing metho d of adjusting renal plasma flow, glomerular filtration rate, and card iac output for interindividual differences in body surface area should be abandoned and replaced by use of the regression method.