Objectives: Serum Cystatin C (CysC) is a promising endogenous marker for estimation of glomerular filtration rate. This study evaluated the analytical performance of the Sysmex CysC assay on the Roche Modular P Chemistry Analyzer (Modular P). Design and methods: The evaluation of imprecision, linearity, recovery, and interference were performed in accordance with the relevant Clinical and Laboratory Standards Institute guidelines protocols, using quality controls and patient samples. Comparison studies were conducted against the Siemens and Roche assay for CysC. Results: At CysC concentrations of 0.45 and 1.80 mg/L, the within-run coefficient of variations (CVs) were 2.81% and 2.04%, respectively, and the between-run CVs were 3.14% and 3.26%. The CysC assay was proven throughout the measuring range from 0.10 to 8.00 mg/L. Good correlations were achieved among the 3 CysC assays. The Sysmex assay appeared to report higher results than the Siemens assay. No interference was detected from hemoglobin 5 g/L, bilirubin (free or conjugated) 200 mg/L, or chyle 1500 formazin turbidity units. Conclusions: The Sysmex CysC assay performed well with regard to basic performance, including precision, dilution linearity, and effects of interfering substances, and is an acceptable assay for the determination of CysC on the Modular P. (C) 2016 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.