Normalization of glomerular filtration rate in children: body surface area, body weight or extracellular fluid volume?

UNLABELLED Glomerular filtration rate (GFR) is usually expressed in relation to body surface area (BSA) for standardization and comparison between individuals of different sizes, although relating it to extracellular fluid volume (ECV) is technically much simpler because the ratio GFR/ECV is almost equal to the rate constant of the second exponential of the 51Cr-EDTA plasma clearance curve. The aim of this study was to investigate the physiological validity of expressing GFR in relation to ECV by comparing it with GFR normalized for BSA and body weight. METHODS GFR was measured from a five-sample 51Cr-EDTA plasma clearance curve. After appropriate scaling, the rate constant of the terminal exponential is equal to GFR/ECV. RESULTS GFR normalized by any of the three variables increased between 0-6 mo, but was less in the case of GFR/weight compared with the other two. From 6 to 24 mo, GFR/BSA continued to increase, whereas GFR/ECV and, in particular, GFR/weight remained relatively constant. It appears that as the child grows to age 6 mo, weight outstrips BSA and ECV but is followed by GFR. From 6 to 24 mo, weight, ECV and GFR increase at a faster rate than BSA. By applying regression analysis to the curves of GFR and its normalization variables versus age, expected (e) and relative (r, i.e., observed/expected) values were obtained for each patient. In the age group 6-24 mo, but not 0-6 mo, r[GFR/ECV] was significantly closer to e[GFR/ECV] than the corresponding comparisons with r and e[GFR/BSA] (p < 0.001) and r and e[GFR/weight] (p < 0.002), supporting the hypothesis that as ECV deviates from the expected value for the patient's age, it tends to be followed by GFR. CONCLUSION Our data show that ECV is the optimal normalization variable in children, particularly those over the age of 6 mo.