A particle-enhanced turbidimetric immunoassay for quantitative determination of orosomucoid in urine: development, validation and reference values.

Increased urinary orosomucoid excretion rate (UOER) is an independent predictor of cardiovascular and all-cause mortality in patients with type 2 diabetes, as demonstrated by a conventional, immunoturbidimetric method. We wanted to optimize the method by developing a fully automated, particle-enhanced turbidimetric (PET) immunoassay with a lower detection limit, to allow assessment of orosomucoid in urine in healthy individuals and patients. A micro-particle-based immunoreagent was prepared for a PET immunoassay. The calibration was traceable to the certified reference material (CRM 470) for specific human serum proteins. We studied 69 healthy adults (28 men and 41 women) to establish reference values for the new assay. The detection limit of orosomucoid in urine was found to be 0.05 mg/l, about 20 times lower than for the conventional assay. Within-run imprecision [CV%, (level)] was 6.7% (0.23 mg/l), 1.0% (1.08 mg/l) and 1.0% (4.69 mg/l). Total imprecision [CV%, (level)] was 10.4% (0.23 mg/l), 3.9% (1.08 mg/l) and 3.4% (4.69 mg/l). Reference values [median (2.5-97.5 percentiles)] for UOER were 0.36 (0.07-2.04) microg/min and for urinary orosomucoid/creatinine ratio 0.04 (0.009-0.17) mg/mmol. We describe a fully automated, transferable, sufficiently precise, high-sensitivity assay for orosomucoid in urine and present reference values traceable to CRM 470.