Abstract
An immunoturbidimetric assay for cystatin C was optimized with respect to assay imprecision. After investigating the optimum pH, polyethylene glycol concentration and specimen volume, two modifications were introduced: an increase in specimen volume to 25µL; and an extension of the pre-incubation period to 240s. These modifications produced an assay with between-batch imprecision (coefficient of variation, n=10 or 11) ranging from 3·9% at 0·72mg/L to 1·3% at 5·29mg/L. The assay was susceptible to interference from lipaemia and haemolysis but not bilirubinaemia in both the original and modified protocol. Extending the preincubation to 240s improved tolerance to common interferences and retained assay applicability in the routine clinical setting.