Prediction of glomerular filtration rate from serum concentration of cystatin C: comparison of two analytical methods.

Background: The aim of this study was to compare two methods used to measure serum cystatin C (Cys) and their accuracy to predict glomerular filtration rate (GFR).
Methods: Three hundred and sixty-seven adult chronic kidney disease (CKD) patients with different functional impairments participated in this study. GFR was determined as the renal clearance of 99mTc-DTPA. Serum concentrations of cystatin C (SCys) were determined with an immunonephelometric method and with an immunoturbidimetric method.
Results: A very high linear correlation was found between the two measurements of SCys (r=0.929). The mean difference of SCysTurb-SCysNeph was 0.02±0.43 mg/L (not significant). A high logarithmic correlation was also found between SCys and GFR (r was 0.919 for SCysNeph and 0.937 for SCysTurb). By means of multiple regression analysis, we developed formulae to predict GFR from SCysNeph, SCysTurb and SCr. For comparison, GFR was predicted using published formulae. A good agreement was found between predicted GFR and measured GFR. The results showed that the accuracy of SCysNeph, SCysTurb and SCr and of the different prediction formulae were quite similar.
Conclusions: The immunoturbidimetric method seems adequate to measure SCys and to predict GFR and its impairment in CKD, at least similar to the immunonephelometric method. The accuracy of SCys and of derived formulae was not higher than that of SCr and SCr-based formulae.