Measurement of urinary cystatin C by particle-enhanced nephelometric immunoassay: precision, interferences, stability and reference range

Background Urinary excretion of the low molecular weight protein cystatin C is a marker of renal disorders and a good predictor of the severity of acute tubular necrosis. We evaluated the measurement of urinary cystatin C and determined its reference range. Methods Measurement of urinary cystatin C by particle-enhanced nephelometric immunoassay (PENIA) in 102 patients with various renal disorders and 133 healthy controls. We assessed the influence of pH and temperature, interferences on urinary cystatin C measurement, as well as cystatin C adsorption to plastic. Results The upper reference value for urinary cystatin C was 0.28 mg/L, independent of age and gender. Accuracy and linearity (r(2)=0.996) were excellent. Intra- and inter-assay precision were less than or equal to4.8% and less than or equal to5.2%, respectively. Albumin (less than or equal to160 g/L), bilirubin (less than or equal to500 mumol/L) and haemoglobin (less than or equal to210 mumol/L) did not show interferences. Urinary cystatin C was stable, at urine pHgreater than or equal to5, at -20degreesC and 4degreesC for 7 days, and at 20 C for 48 h. Freezing and thawing did not influence urinary cystatin C concentration. There was no adsorption of cystatin C to plastic. Conclusion Urinary cystatin C measurement by PENIA is precise. High stability and no interference add to the practicability of urinary cystatin C as a routine biochemical test.