Measurement and validation of urinary cystatin C by particle-enhanced turbidimetric immunoassay on Architect ci8200

Sammanfattning: Cystatin C, a 13 kDa low molecular weight protein is an inhibitor of cysteine proteases. Due to its low molecular weight and positive charge at physiological pH, it is freely filtered by the glomerulus and catabolized after reabsorption by proximal tubular cells with a low concentration (0.03-0.3 mg/L) in urine amongst healthy subjects. Urinary cystatin C is a potential biomarker detection of acute kidney injury (AKI) in the acute phase when patients are submitted to the intensive care unit. The aim in this report was to perform a full method validation of urinary analysis of cystatin C on a high throughput chemical analyzer by particle-enhanced turbidimetric immunoassay (PETIA) at the University Hospital in Uppsala, Sweden. The antigen excess, linearity, lower limit of quantification (LoQ), recovery, assay precision, stability and interference caused by haemoglobin was evaluated. No hook effect was observed, the assay was linear over the studied interval <0.001-0.950 mg/L with a regression of R2=0.9994. The LoQ was calculated to 0.020 mg/L with a coefficient of variation (CV) ≤10% which was considered acceptable. The assay had a recovery between 93-100% and the assay precision had a total CV <3.5%. Cystatin C is stable for 3 days in room temperature and 14 days in +4C. The assay did not show any major interference with haemoglobin. The urinary cystatin C showed good precision and performance characteristics by measurements using PETIA all of which is a necessary qualification for a biomarker at a 24-h running routine laboratory.