Indexed glomerular filtration rate as a function of age and body size

The conventional way in which to scale or index a measurement of glomerular filtration rate (GFR) is to express it in relation to body surface area (BSA). However, BSA may not be appropriate for infants and children because, as individuals increase in size, their relative BSA decreases. Several other whole-body variables have been suggested as alternatives, including extracellular fluid volume (vECF). The purpose of the present study was to compare BSA and vECF as variables against which to index GFR, and in particular to look at this comparison in children versus adults. A total of 130 patients (age range 1–80 years; 40 patients < 12 years) undergoing clinically indicated routine measurement of GFR using the bolus-injection single-compartment technique were included in the study. GFR was measured as the plasma clearance of [⁵¹Cr]EDTA as assessed from three peripheral venous blood samples taken between 2 and 4 h after injection of [⁵¹Cr]EDTA. Volume of distribution (V_d) was obtained by extrapolation of the clearance curve to zero time. GFR was scaled to a BSA of 1.73 m². GFR and GFR/1.73 m² were corrected to account for the assumption of a single compartment. The rate constant of the exponential between 2 and 4 h was also corrected to give GFR/litre ECF. GFR and GFR/1.73 m² were both divided by GFR/litre ECF, to give vECF and vECF/1.73 m² respectively. Weight per unit BSA increases as a linear function of BSA. vECF is always less than V_d, on average by about 30%. vECF increased as an exponential function of BSA and as a linear function of body weight. vECF/70 kg body weight was higher in children (16.2±3 litres) than adults (13.4±2.3 litres), but vECF/1.73 m² was lower in children (9.7±1.7 litres) compared with adults (12.4±2 litres). vECV/1.73 m² increased as a function of both age and BSA, but vECF/kg decreased. GFR/12.5 litres vECF was higher than GFR/1.73 m² in children, but these values were similar in adults, with the ratio of these two forms of indexed GFR falling significantly with both age and BSA. Although this was not a normal population, but one with a wide range of renal function, GFR/vECF showed a strong inverse association with age, whereas for GFR/BSA the association was weak. In conclusion, these data provide further evidence that vECF is more valid physiologically for indexing GFR than is BSA, especially in children. Nevertheless, a GFR measurement in a child should ideally be expressed as a percentage of normal for that child's age. However, such normal values are not yet available.