Abstract
Sir, We read with interest the study by Poge et al. evaluating a refit version of the MDRD equation and the Mayo Clinic Quadratic equation to improve estimation of GFR in renal allograft recipients [1]. As pointed out by Poge et al., these two equations were not developed using a standardized assay for serum creatinine. Because the same creatinine assay was used to develop these two equations, it still could be shown that differences in the source population can lead to different equations [2]. However, we recognize that it has been difficult to determine to what extent the Mayo Clinic Quadratic and MDRD equations perform differently due to assay calibration versus the source population. A confirmed conversion factor between a standardized assay (i.e. traceable to isotope dilution mass spectroscopy) and the Mayo Clinic assay used to develop these equations may be helpful. Since the Mayo Clinic assay used an uncompensated rate-Jaffe method, the following calibration correction based on assay differences reported in the literature could be used [3]:
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