How to estimate GFR in children.

The reference method to determine glomerular filtration rate (GFR) in children is the inulin clearance. Similarly, Cr51-EDTA, iohexol and iothalamate can be used as exogenous GFR markers [1,2]. Exogenous markers are expensive and rather impractical. Creatinine is by far the most commonly used biochemical marker of renal function. The commonest principle for assaying creatinine is the Jaffe reaction [3]. Since Jaffe only observed a complexation between picric acid and creatinine in alkaline environment and never described an analytical method, variation amongst ‘Jaffe method’ recipes is broad [4]. The analytical bias of current creatinine methods (due to interference by pseudochromogens and calibration differences) is still disappointing: the compensated Jaffe method shows a small positive bias, whereas a major positive bias is observed for the dry chemistry and the uncompensated Jaffe methods [5]. Interlaboratory variation for creatinine is still unacceptably high; recent studies have reported median method group variation coefficients of 6.4% at a concentration of 80μmol/L [6]. Such variation leads to an unacceptable variation in the estimation of kidney function in young children and infants. Equations to estimate GFR require knowledge of the calibration of the serum creatinine assay [7].