Effect of creatinine assay standardization on the performance of Cockcroft–Gault and MDRD formula in predicting GFR

Sir, The recent article of van Biesen et al . [ 1 ] illustrates the importance of standardization of serum creatinine assays when using formulas to estimate glomerular filtration rate (GFR). We provide further proof of the importance of standardization of creatinine. We previously compared the performance of the modification of diet in renal disease (MDRD) formula and the Cockcroft–Gault (CG) formula in healthy persons and normo-albuminuric diabetic patients [ 2 ]. As reported in this Journal, we concluded that the MDRD equation was less accurate than CG formula in predicting GFR (measured by inulin clearance). Our data were based upon a creatinine assay, based on the Jaffé kinetic reaction performed on a Hitachi 747 auto-analyser. Meanwhile it has been shown that correct use of the MDRD equation necessitates calibration of the creatinine assay against the assay used by the Cleveland Clinic Laboratory [ 3 ]. The Cleveland Clinic Laboratory originally used a Beckman modified kinetic rate Jaffé reaction. Recently, samples of the MDRD study were re-analysed and compared with a creatinine assay using isotope dilution mass spectrometry (IDMS) as the gold standard [ 4 ]. IDMS results were highly correlated to enzymatic creatinine, measured by Roche technology. Therefore, this enzymatic assay should preferably be used in the MDRD equations. Results are given in Table 1 . It is apparent that the use of standardized creatinine assay has a major influence on the reported differences between the formula-derived GFR and the true GFR. In fact, in contrast to our earlier finding, our recalculated data indicate that the MDRD formula is probably more accurate than the CG formula, also in persons with normal GFR. Reporting of MDRD-GFR using creatinine assays that are not calibrated using a gold standard should be discouraged. Conflict of interest statement . None declared. Measured and predicted GFR in healthy subjects and diabetes patients without [ 2 ] and with standardization of creatinine assay Values are expressed as means ± SD; GFR, glomerular filtration rate (ml/min/1.73 m 2 ); CG, prediction of GFR with Cockcroft–Gault formula (ml/min/1.73 m 2 ); MDRD, prediction of GFR with the MDRD formula (ml/min/1.73 m 2 ). Measured and predicted GFR in healthy subjects and diabetes patients without [ 2 ] and with standardization of creatinine assay Values are expressed as means ± SD; GFR, glomerular filtration rate (ml/min/1.73 m 2 ); CG, prediction of GFR with Cockcroft–Gault formula (ml/min/1.73 m 2 ); MDRD, prediction of GFR with the MDRD formula (ml/min/1.73 m 2 ).