Correct evaluation of renal glomerular filtration rate requires clearance assays

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Abstract
Iohexol clearance is an accepted, but time-consuming assay for the measurement of glomerular filtration rate (GFR). We investigated if simpler methods could predict GFR. Sixty-nine children with hematological-oncological disorders participated. A linear relationship was established by regression analysis between iohexol clearance (n=734) and 1/s-creatinine (r=0.45, n=727), s-cystatin C (r=0.41, n=518), and the Schwartz (r=0.45, n=723), Counahan-Barratt (r=0.48, n=723), and modified Counahan-Barratt formulae (r=0.48, n=723). These correlations improved when one GFR measurement per individual was compared with each of the five parameters. We further investigated if iohexol clearance could accurately be replaced. The degree of variation in predicting GFR was estimated by the standard deviation of the residuals (Sres). For 1/s-creatinine and s-cystatin C, Sres was 39 and 38 ml/min per 1.73 m2. For the formulae of Schwartz, Counahan-Barratt, and modified Counahan-Barratt, the Sres was 43, 40, and 40 ml/min per 1.73 m2, respectively. The wide variations of the Sres were not reduced when one GFR measurement per child was compared with the five parameters. Due to the large deviation in predicting GFR, we conclude that the five alternative methods studied cannot replace iohexol clearance for measurement of GFR.

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