NDT Advance Access originally published online on January 19, 2008
Nephrology Dialysis Transplantation 2008 23(3):1065-1066; doi:10.1093/ndt/gfm837
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Reply
E-mail: shoker{at}sask.usask.caSir,
We thank Delanaye et al. for raising certain remarks on our methodology that led, in their conclusion, to an analytical limitation of our study. These limitations pertain to a standardization of serum creatinine and the ELISA method used to measure cystatin C. They concluded that more studies seem necessary on this topic, with improved methodology, notably from an analytical point of view.
As for a standardization of serum creatinine, we believe that this is not the case obviously for all equations other than the MDRD equation. MDRD2 (IDMS) has been suggested as an alternative for labs that do not standardize their method to measure serum creatinine. We therefore included the modified MDRD2 (IDMS) for this purpose. We hope that we have emphasized this limitation and presented an argument from the literature (Hallan et al., Am J Kidney Dis 2004; 44: 84), who pointed out that the bias due to a missing calibration decreases as serum creatinine increases, as in kidney transplant patients.
A more disconcerting limitation is the method used to measure cystatin C. We have been assured by BioVendor that in this ELISA method, more recently developed antibodies are used. This newer ELISA method has been used reliably by other investigators [1–3]. As indicated, this method has an R value of 0.97, which translates to R2 of 0.94 when compared with the DADE B latex-assisted turbidimetry method and R2 of 0.92 with the DAKO ELISA method. In our judgment, this leaves little margin for error. We agree that the coefficient of variation (CV) of this method is limited. This is likely due to the enhanced sensitivity of this ELISA technique.
Looking at the results of our paper, and those presented in the literature, we certainly find significant difficulty to assume that the drawbacks of our techniques would significantly explain the marked limitation of GFR estimators to predict GFR. We agree that improved techniques will give accurate results but we disagree with the suggested statement that these analytical concentrations can explain the limited performance of the tested equations.
Conflict of interest statement. None declared.
Saskatchewan Transplant Program, University of Saskatchewan, Saskatchewan, Canada
References
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