Body surface area correction in single-sample methods of glomerular filtration rate estimation

Single-sample methods of estimating glomerular filtration rate (GFR) are used routinely, in a number of institutions, to calculate values of total plasma clearance (TPC), which are then converted to GFR. Adjustment of the final estimate for body surface area (BSA) is not adequate to allow adult equations to be used in children and work has been undertaken to seek an age-independent technique. The aim of this study was to evaluate the ability of an alternative method of correcting for BSA, of scaling the plasma sample activity, to achieve an age-independent solution. The accuracy of the scaled TPC values was compared with a monoexponential analysis as the 'gold' standard. Data from 60 patients were analysed using 16 single-sample models. Patients were chosen so that 10 could be allocated to each of six categories involving three age groups (0-3 years, 4-17 years and 18+ years) and two classes of TPC (< 40 and > 40 ml.min-1). Adjustment of the plasma activity gave more accurate results than correction of the final estimate in 87% of the categories for 15 of the models. This improvement was particularly apparent in the case of adult equations applied to child data, which gave results similar to those obtained using child equations, indicating that they can be used with equal accuracy.