Validation of a new plasma cystatin C-based formula and the Modification of Diet in Renal Disease creatinine-based formula for determination of glomerular filtration rate.

OBJECTIVE: New proposed definitions of chronic kidney disease necessitate the development and use of simple and accurate methods for estimating glomerular filtration rate (GFR). Plasma cystatin C has been shown to be a more reliable GFR marker than creatinine and formulae for estimating GFR have been reported. The purpose of this study was to validate a cystatin C-based GFR prediction equation in a different population from the derivation set but using the cystatin C assay of a single laboratory, and to compare the results with that of the creatinine-based Modification of Diet in Renal Disease (MDRD) Study equation.

MATERIAL AND METHODS: A newly presented formula based on plasma cystatin C and gender and the MDRD formula based on creatinine for estimation of GFR were validated in an unselective patient material. Single sample iohexol clearance was used as the GFR reference method in 406 consecutive patients with GFR varying from normal to poor renal function. The creatinine assay used was standardized to express true plasma creatinine.

RESULTS: Median bias (1.1%) and accuracy (79.1% of the estimates within +/-30% of iohexol clearance) of the cystatin C formula were close to the derivation set. The accuracy was significantly higher than that of the original four-variable MDRD equation (73.2%; median bias 9.8%). However, the accuracy did not differ significantly from that of the re-expressed MDRD formula (79.6%; median bias 3.2%) based on true creatinine. Both formulae performed with a low bias and acceptable accuracy up to a GFR of 90 ml/min/1.73 m(2).

CONCLUSIONS: GFR estimation based on plasma cystatin C performed equally well in the validation as in the derivation set, and was as accurate as the re-expressed MDRD creatinine-based equation.