Indexing glomerular filtration rate to suit children

In order to be able to compare individuals of differing size, glomerular filtration rate (GFR) is conventionally indexed to body surface area (BSA). This does not, however, suit children because they naturally have a relatively high BSA simply because of their small size. The aim of the study was to identify an appropriate simple whole-body variable based on height and weight suitable for indexing GFR that would be simultaneously appropriate for both children and adults.

A database of 532 routine clinical GFR measurements, each based on 3 venous blood samples obtained between 2 and 4 h after injection of (51)Cr-ethylenediaminetetraacetic acid, was analyzed to give GFR and, using only the half-time of the slope of the clearance curve, the quotient GFR to extracellular fluid volume (ECV). BSA was obtained from the Haycock formula, which is based on height and weight raised to indices to give units of area. Both GFR and GFR/ECV were corrected for the 1-compartment assumption using previously published empiric correction formulas. ECV was obtained by dividing GFR by GFR/ECV. An equation analogous to Haycock's was derived in which the indices of height and weight were varied to give an iterative best fit to ECV instead of BSA.

GFR, ECV, and BSA increase as functions of age until about age 13 y, corresponding to a BSA of about 1.35 m(2), which was taken as the cutoff point between children and adults. As humans grow, their ratio of height to effective radius changes as a nonlinear function of surface area. Humans must therefore change shape as they grow. Moreover, the ECV-to-weight ratio decreases as a function of body size, suggesting that humans also change body composition as they grow. The new equation, giving an iterative best fit to ECV, was ECV = weight(0.6469) x height(0.7236) x 0.02154. ECV, either measured or estimated from the new equation, corresponding to a BSA of 1.73 m(2), was 12.9 L. Expressed as values normalized to the corresponding average adult values, the new equation and the second-order polynomial fit to ECV were superimposed as they increased as functions of BSA or weight. In contrast, normalized BSA and normalized weight were respectively larger and smaller than normalized ECV in children. GFR indexed to the new equation correlated more closely with GFR indexed to ECV than did GFR indexed to BSA and, along with GFR/ECV, showed a greater fall as a function of age than did GFR/BSA.

When required in absolute units rather than as a rate of turnover of ECV, GFR is appropriately indexed to indices of height and weight as defined by this new equation, which avoids disadvantages to children from indexing to BSA. This unmasks higher values of filtration function in children than have hitherto been recognized.