NDT Advance Access originally published online on April 3, 2008
Nephrology Dialysis Transplantation 2008 23(7):2425-2426; doi:10.1093/ndt/gfn136
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S-cystatin C formulae or combination of s-cystatin C and s-creatinine formulae do not improve prediction of GFR
Correspondence and offprint requests to: E-mail: klinef{at}am.centrum.plSir,
We read with great interest the original article by Tidman et al. [1] on the development and validation of the new Orebro-cyst formulae for GFR estimation based on cystatin C serum concentration and combination (mean) of the new formulae and the MDRD equation. The Orebro-cyst equations are constructed using the calculated production rate and extra-renal clearance of cystatin C. Two equations are formulated for different methods of cystatin C determination (DAKO and Gentian). In the aforementioned paper, a formula that combines MDRD and Orebro-cyst provided a greater accuracy than formulae based on s-creatinine or s-cystatin C alone. We have investigated whether the new Orebro-cyst (Gentian) equation or its combination with MDRD can improve proportion of correctly classified patients for CKD stages in our cohort.
Our study was performed on 100 Caucasian subjects: 57 CKD patients, 28 kidney transplant patients and 15 volunteers. In all cases GFR was measured as the plasma clearance of iohexol (iGFR). Creatinine was determined by an enzymatic method (Randox) and cystatin C by PENIA (Dade Behring). The median (range) of the measured GFR was 22 (7–124) ml/min/1.73 m2. The estimated GFR (eGFR) was calculated based on creatinine concentration by the Cockcroft–Gault formula corrected to body surface area (CG/BSA), the abbreviated MDRD equation (aMDRD) and Mayo Clinic quadratic equation, and based on cystatin C serum concentration by Hoek, Larsson and Orebro-cyst (Gentian) equations. The performance of these formulae was analysed according to recommendations in the NKF K/DOQI guidelines. Table 1 shows percentage of patients correctly classified for the different stages of CKD based on the calculated eGFR.
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In our study group the most accurate results were obtained with the Mayo Clinic quadratic equation [2]. Results were within 30% of iGFR in 85% and within 50% of iGFR in 95% of the cases. The median (range) of eGFR was 19.50 (7–141) ml/min/1.73 m2. The bias was 1.6 ml/min/1.73 m2 and precision (±1.96 x standard deviation from difference) –16.6–19.8 ml/min/1.73 m2. The Orebro-cyst formula was significantly less accurate than the Mayo Clinic equation. Results were within 30% of iGFR in 63% and within 50% of iGFR in 82% of the cases. The median (range) of eGFR was 18.00 (4–168) ml/min/ 1.73 m2. The bias was 6.0 ml/min/1.73 m2 and precision –24.2–36.1 ml/min/1.73 m2.
The formula created by combination of the Orebro and aMDRD equation was more precise than Orebro or Mayo Clinic alone (–13.5–14.3 ml/min/1.73 m2) and less biased (0.4 ml/min/1.73 m2), but accuracy was not better than Mayo Clinic (within 30% of iGFR in 79% and within 50% of iGFR in 95% of the cases). The proportion of correctly classified patients was not better than for the Mayo Clinic formula.
To our knowledge there is no previous report on evaluation of Orebro-cyst or combination of the Orebro-cyst and MDRD equation in independent patients group. Unlike Tidman et al., we could not show superiority of combination of both s-creatinine and s-cystatin C formulae for GFR estimation in our patients. In our opinion, none of the evaluated equation was suitable for GFR estimation in diverse population. All of them may cause misclassification in a selected group of patients.
Conflict of interest statement. The results presented in this paper have not been published previously in whole or part, except in abstract form.
aw Wozniak1
1 Department of Clinical Chemistry Wroclaw Medical University Pasteura 2, 50-367 Wroclaw 2 Department of Nephrology and Transplantation Medicine, Wroclaw Medical University, Traugutta 57/59, 50-417 Wroclaw 3 Department of Laboratory Diagnostics, University Hospital No. 1, Pasteura 2, 50-367 Wroclaw Poland
References
- Tidman N, Sjöström P, Jones I. A comparison of GFR estimating formulae based upon s-cystatin C and s-creatinine and combination of the two. Nephrol Dial Transplant (2008) 23:154–160.
[Abstract/Free Full Text] - Rule AD, Larson TS, Bergstralh EJ, et al. Using serum creatinine to estimate glomerular filtration rate: accuracy in good health and chronic kidney disease. Ann Intern Med (2004) 141:929–937.
[Abstract/Free Full Text]
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