Recalibration of cystatin C using standardized material in Siemens nephelometers

Abstract

Background: Cystatin C is a key GFR biomarker. Recently, Siemens recalibrated the assay based on certified reference material ERM-DA471/IFCC. The NIH-funded longitudinal chronic kidney disease in children (CKiD) study has > 3000 cystatin C measurements based on a pre-IFCC calibrator provided by Siemens. Since cystatin C values for CKiD are now standardized to IFCC certified reference material, it is important to relate the IFCC-calibrated results to the previous values so that there are no discontinuous results.

Methods: We diluted cystatin C ERM-DA471/IFCC (5.48 mg/L) into buffer and compared results with predicted ones. We then updated the cystatin C application on our BN II nephelometer to provide results based on pre-IFCC and IFCC calibrations of CKiD specimens simultaneously. We assayed 51 previously analyzed sera and 62 fresh additional specimens.

Results: The predicted concentrations from the IFCC standard were consistently 17% higher than the measured values using the pre-IFCC calibration (y = 1.1686x). Similarly, the re-run and fresh sample concentrations were 17% higher via the IFCC calibration than by the pre-IFCC calibration (y = 1.168x). There was very high reliability in the measurements using the previous calibration for re-run specimens (0.99) and for 33 pristine specimens using IFCC calibration (0.99).

Conclusions: We confirm the recalibration proposed by Siemens. To convert pre-IFCC results to IFCC-calibrated concentrations, the value is multiplied by 1.17. Conversely, one divides IFCC-calibrated results by 1.17 to estimate GFR via previously published pre-IFCC CKiD eGFR equations. For older adolescents, cystatin C has already been standardized and can be directly applied to the CKD-EPI equations.

Keywords: CKiD study; Calibration; Cystatin C; Glomerular filtration rate; Nephelometry; Reference.

Figures

Figure 1

Pre-IFCC calibrated measurements plotted against dilutions of ERM-DA471/IFCC reference material in PBS. Most measurements were performed in duplicate and the average value utilized. Including the reference material used at full strength (5.48 mg/L), there are 20 predicted values over the physiologic range plotted for the relationship. The line pre-IFCC = 0.856 x IFCC reference) depicts the excellent fit of measurements via the pre-IFCC calibration method to the dilutions of the reference material.

Figure 2

Comparison of serum specimens simultaneously measured using pre-IFCC and IFCC calibrated methods on a Siemens BN II. Horizontal lines indicate 51 specimens previously run slightly over a year previously with the line spanning the two values of the old (pre-IFCC) calibration. Diamonds identify 62 pristine specimens measured using pre-IFCC and IFCC calibrations. Open squares indicate values from GJS measured concomitantly with each run of 10–20 sera. The line describes the relationship between old and new calibrations (IFCC = 1.168 x pre-IFCC calibration); the intercept was not significant.