NDT Advance Access originally published online on February 18, 2008
Nephrology Dialysis Transplantation 2008 23(5):1763; doi:10.1093/ndt/gfn024
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Need of standardization for creatinine and cystatin C-based GFR equations in renal transplantation
Correspondence and offprint requests to: E-mail: dr.poege{at}nephrologie-bonn.deSir,
We read with interest the detailed comparison of various creatinine- and cystatin C (Cys C)-based equations to determine GFR in patients after kidney transplantation [1]. This publication provides a comprehensive overview of the most commonly used equations and importantly, applies inulin clearance as gold standard GFR measurement. The authors conclude that creatinine-based formulae are not inferior to Cys C-based estimations of GFR.
However, we do have some concerns and would like to draw attention to the fact that neither the creatinine determination calibrated method (e.g. IDMS method) nor the accepted methods for Cys C determination (Dade Behring or DAKO) were used. Although the lack of calibration is discussed as a potential drawback, the consequences of the presented results are not outlined.
As demonstrated in a recent publication, even a very small bias from the IDMS-creatinine (+1.15 µmol/l) results in a considerable modification of bias, precision and accuracy [2]. To provide a mathematical example for the MDRD equation, an overestimation of 5 µmol/l, at a creatinine level of 100 µmol/l, leads to an increase in one of the multipliers of the MDRD equation: [105/88.4]–1.154 = 0.82 instead of [100/88.4]–1.154 = 0.867. This means, for a 50-year-old Caucasian male, the MDRD equation will yield 64.8 ml/min/1.73m2 instead of 68.6 ml/min/1.73m2. Surely this is of limited clinical impact; however, it will affect the statistical results of the above-mentioned publication considerably.
In regard to the Cys C-based equations, it should be noted that virtually all of the applied equations are based on Dade Behring or DAKO assays which were not applied in this study. Application of a different Cys C determination technique introduces a bias affecting the performance of the Cys C-based estimation. Naturally, even a high correlation of the applied Cys C method with both assays does not exclude a systematic over- or underestimation.
Thus, a valid conclusion is hampered if non-IDMS-calibrated creatinine-based equations are compared with formulae based on non-standardized Cys C.
In conclusion, this work is only interesting for transplant centres that determine creatinine by a SYNCHRON LX 20 system and Cys C by ELx808TM absorbance microplates. Outside of these laboratory settings, the presented data are of limited usefulness.
Conflict of interest statement. None declared
1 Department of Internal Medicine I 2 Department of Clinical Biochemistry, University Hospital Bonn, Germany
References
- Zahran A, Qureshi M, Shoker A. Comparison between creatinine and cystatin C-based GFR equations in renal transplantation. Nephrol Dial Transplant (2007) 22:2659–2668.
[Abstract/Free Full Text] - Poge U, Gerhardt T, Stoffel-Wagner B, et al. Can modifications of the MDRD formula improve the estimation of glomerular filtration rate in renal allograft recipients? Nephrol Dial Transplant (2007) 22:3610–3615.
[Abstract/Free Full Text]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||