Expressing the modification of diet in renal disease study equation for estimating glomerular filtration rate with standardized serum creatinine values

Purpose: We sought to reexpress the 4-variable Modification of Diet in Renal Disease (MDRD) Study equation for estimation of glomerular filtration rate (GFR) using serum creatinine (S_cr) standardized to reference methods. Methods: Serum specimens included creatinine reference materials prepared by the College of American Pathologists (CAP), traceable to primary reference material at the NIST, with assigned values traceable to isotope dilution mass spectrometry (IDMS), a calibration panel prepared by the Cleveland Clinic Research Laboratory (CCRL), and frozen samples from the MDRD Study. Split specimens were measured at the CCRL using the Roche enzymatic and Beckman CX3 kinetic alkaline picrate assays. Results: Roche enzymatic assay results on CAP samples were comparable to IDMS-assigned values. Beckman CX3 assay results in 2004-2005 were significantly higher than but highly correlated with simultaneous Roche enzymatic assay results (r² = 0.9994 on 40 CCRL samples) and showed minimal but significant upward drift from Beckman CX3 assay results during the MDRD Study in 1989-1991 (r² = 0.9987 in 253 samples). Combining these factors, standardized S_cr = 0.95 x original MDRD Study S_cr. The reexpressed 4-variable MDRD Study equation for S_cr (mg/dL) is GFR = 175 x standardized S_cr ^-1.154 x age^-0.203 x 1.212 (if black) x 0.742 (if female), and for S_cr (μmol/L) is GFR = 30849 x standardized S _cr^-1.154 x age^-0.203 x 1.212 (if black) x 0.742 (if female) [GFR in ml·min^-1·(1.73 m²) ^-1]. Conclusion: When the calibration of S_cr methods is traceable to the S_cr reference system, GFR should be estimated using the MDRD Study equation that has been reexpressed for standardized S _cr.