A sol particle homogeneous immunoassay for measuring serum cystatin C

Objectives: We have developed a sol particle immunoassay (SPIA) for measuring serum cystatin C, an endogenous marker of glomerular filtration rate (GFR). Design and methods: We used colloidal gold particles coated with anti-cystatin C antibodies. Results: The assay was linear in the range 0.2 to 8 mg/L and showed good correlation between theoretical and obtained values. The within and between-day coefficients of variation (CV) varied from 1.1 to 1.6% and 0.4 to 1.0%, respectively. Analytical recovery was 95.7 to 103.7%. No interference could be detected from bilirubin (up to 200 mg/L), hemoglobin (up to 3 g/L), chyle (up to 5000 FTU), rheumatoid factor (up to 1000 IU/mL) or anticoagulants. Serum samples (n = 101), from which turbidity had been removed, were measured either with our assay or with Dako Cystatin C PET kits, using a Model 7070 Hitachi automatic clinical analyzer. Comparing these two methods, the calculated linear regression equation and the correlation coefficient were Y = 0.986 x −0.153 and r = 0.995, respectively. Conclusions: Our new SPIA assay is a fully automated, homogeneous immunoassay that can readily be used in conjunction with various commercial analyzers that are currently available. The assay is sensitive, precise and suitable for clinical use and appears to offer advantages over other GFR markers such as creatinine.