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Why the MDRD equation should not be used in patients with normal renal function (and normal creatinine values)?

Sir, – Using creatinine-based formulae for glomerular filtration rate (GFR) has become a classic clinical, medical practice. American K/DOQI guidelines favor the formula developed by Levey et al. [1999, 2002] from their “Modified Diet in Renal Diseases” (MDRD) study. However, several authors have underlined the inaccuracy of this formula when applied to patients with normal renal function [Cirillo et al. 2005, Froissart et al. 2005, Hallan et al. 2004, Lin et al. 2003]. Actually, all GFRs estimated to be above 60 ml/min/1.73 m2 must be reported as > 60 ml/min/1.73 m2, without absolute precision of the value [Levey et al. 2005b, Murthy et al. 2005]. One explanation of the inaccuracy is the variation in the serum creatinine assay calibration [Coresh et al. 2002, Lamb et al. 2005, Murthy et al. 2005]. The problems linked to calibration may be overcome using a traceable (gold standard) serum creatinine measurement (isotope dilution mass spectrometry) as recently suggested [Lamb et al. 2005, Levey et al. 2005a, Myers et al. 2006]. The lack of accuracy of the MDRD formula in a non-renal population can also be explained by the fact that the MDRD formula has been elaborated exclusively from a renal failure population and that the relationship between creatinine and GFR are not the same in normal and renal failure populations [Bostom et al. 2002, Froissart et al. 2005]. We think that a third explanation may be valid. In fact, the precision of Jaffe methods can also contribute to the inaccuracy of creatinine-based formulae in normal GFR ranges [Myers et al. 2006]. In our laboratory, we use the compensated kinetic Jaffe method for creatinine measurement (Modular, Roche Diagnostics, Mannheim, Germany). The analytical coefficient of variation (CVa) for low creatinine values is 5.5%. The intra-individual coefficient of variation (CVi) for creatinine is 4.3% [Ricos et al. 1999]. The critical difference (= 1.414 × 1.96 × (CVa 2 + CVi 2)0.5), defined as the smallest change in results which is not due to chance, is, thus, 19% for serum creatinine [Costongs et al. 1985]. A creatinine value of 1 mg/dl is, thus, not different from 1.19 or 0.81 mg/dl. When the two latter values are introduced into the simplified MDRD formula for a man of 60 years, the results are 66.3 and 103.3 ml/min/1.73 m2, respectively. The absolute differences between these estimated GFRs and GFRs estimated with 1 mg/dl which is 81 ml/min/1.73 m2 are 15 and 22 ml/min/1.73 m2! If the Cockcroft formula is used [Cockcroft and Gault 1976] for a 60-year-old man weighing 80 kg, the same process will produce similar absolute differences of 14 and 21 ml/min. These potential differences are still greater for creatinine lower than 1 mg/dl because the creatinine – GFR relationship is exponential [K/DOQI 2002] and CVa rises with decreasing creatinine values. Potential differences of creatininebased formulae associated with potential variation in creatinine measurement which are only due to chance (CVa and CVi) are thus not negligible. The critical difference can be decreased with better CVa. If improvements are expected regarding standardization of creatinine calibration, sufficient improvement seems difficult to obtain with the precision of Jaffe methods. In fact, if a CVa of 2% can be reached [Froissart et al. 2005], the critical difference will decrease to 13% but differences in MDRD results will still be of significance (14 and 11 ml/min/1.73 m2).