Effect of Two Cystatin C Reagents and Four Equations on Glomerular Filtration Rate Estimations After Standardization

BACKGROUND: Serum cystatin C (cysC), which is less affected by sex, race, and muscle mass than creatinine, is a useful biomarker of the estimated glomerular filtration rate (eGFR). The standardization of cysC measurements remains controversial, although a certified reference material (ERM-DA471/IFCC) is available. Moreover, the effect of combinations of cysC reagents and equations for eGFR is unclear. METHODS: We conducted a simulation analysis of cysC measured using two reagents standardized against ERM-DA471/IFCC—Gentian cystatin C immunoassay (Gentian(cys); GentianAS, Moss, Norway) and Roche Tina-quant Cystatin C Gen.2 (Roche(cys); Roche, Mannheim, Germany)—on a Cobas c702 system (Roche) and eGFR generated by eight combinations of four equations 2012 cystatin C-based Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI(cys)); the Caucasian, Asian, pediatric, and adult equation (CAPA(eq)); full age spectrum equation (FAS(eq)); and 2023 cystatin C-based European Kidney Function Consortium equation (EKFC(cys)). RESULTS: A total of 148 participants (mean age, 60.5±14.5 years; 43% female) were enrolled. The mean cysC was 1.72±1.44 mg/L for Gentian(cys) and 1.71±1.35 mg/L for Roche(cys). Regression analysis showed concordance between the reagents within 0.85–4.40 mg/L when using ±7.61% total allowable error. Lin’s concordance correlation coefficient of eGFR, by combining the measuring system and equation, varied from 0.73 to 1.00. CONCLUSIONS: The equivalence of cysC values at low concentrations (<0.85 mg/L) between the two reagents was unsatisfactory. Results obtained with different measurement systems could lead to larger differences in eGFR varying with the combination.