Recalibration of cystatin C using standardized material in Siemens nephelometers
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.
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.
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).
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.
KeywordsCystatin C Glomerular filtration rate Nephelometry CKiD study Reference Calibration
Data in this manuscript were collected by the chronic kidney disease in children (CKiD) prospective study with clinical coordinating centers at Children’s Mercy Hospital (BAW and the University of Missouri—Kansas City (BAW) and the Children’s Hospital of Philadelphia (SLF), Data Coordinating Center at the Johns Hopkins Bloomberg School of Public Health (AM, DN), and the Central Biochemistry Laboratory at the University of Rochester Medical Center (GJS).
The CKiD study is funded by the National Institute of Diabetes and Digestive and Kidney Diseases, with additional funding from the National Institute of Neurological Disorders and Stroke, the National Institute of Child Health and Human Development, and the National Heart, Lung, and Blood Institute (U01-DK66143, U01-DK66174, U24-DK66116, U24-DK82194). The CKiD website is located at http://www.statepi.jhsph.edu/ckid.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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