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 (B SA). However, BSA may not be appropriate for infants and children because, as individuals increase in size, their relative BSA decreases. Several othe r whole-body variables have been suggested as alternatives, including extra cellular fluid volume (vECF). The purpose of the present study was to compa re 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 patien ts (age range 1-80 years; 40 patients < 12 years) undergoing clinically ind icated routine measurement of GFR using the bolus-injection single-compartm ent technique were included in the study. GFR was measured as the plasma cl earance of [Cr-51]EDTA as assessed from three peripheral venous blood sampl es taken between 2 and 4 h after injection of [Cr-51]EDTA. Volume of distri bution (V-d) was obtained by extrapolation of the clearance curve to zero t ime. GFR was scaled to a BSA of 1.73 m(2). GFR and GFR/1.73 m(2) were corre cted to account for the assumption of a single compartment. The rate consta nt of the exponential between 2 and 4 h was also corrected to give GFR/litr e ECF. GFR and GFR/1.73 m(2) were both divided by GFR/litre ECF, to give vE CF and vECF/1.73 m(2) respectively. Weight per unit BSA increases as a line ar 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 o f body weight. vECF/70 kg body weight was higher in children (16.2 +/- 3 li tres) than adults (13.4 +/- 2.3 litres), but vECF/1.73 m(2) was lower in ch ildren (9.7 +/- 1.7 litres) compared with adults (12.4 +/- 2 litres). vECV/ 1.73 m(2) increased as a function of both age and BSA, but vECF/kg decrease d. GFR/12.5 litres vECF was higher than GFR/1.73 m(2) in children, but thes e values were similar in adults, with the ratio of these two forms of index ed 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 s howed a strong inverse association with age, whereas for GFR/BSA the associ ation was weak. In conclusion, these data provide further evidence that vEC F is more valid physiologically for indexing GFR than is BSA, especially in children. Nevertheless, a GFR measurement in a child should ideally be exp ressed as a percentage of normal for that child's age. However, such normal values are not yet available.