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Development and validation of a particle-enhanced turbidimetric inhibition assay for urine albumin on the Dade aca^® analyzer

¹ Department of Clinical Biochemistry, St Bartholomew's and the Royal London School of Medicine and Dentistry, Turner Street, London E1 2AD, UK.

² Glasgow site, Dade International, Wilmington, DE.
^a Author for correspondence. Fax +44 171 377 1544; e-mail d.newman{at}mds.qmw.ac.uk

The measurement of urine albumin now has a well-established role in the monitoring of patients with diabetes mellitus. We have developed a particle-enhanced immunoturbidimetric inhibition assay for urine albumin on the Dade aca^® analyzer. The inhibition approach removes any of the potential antigen excess difficulties that could be expected from the wide clinical range of urine albumin, but retains the sensitivity advantages of latex-enhanced immunoturbidimetry. Human serum albumin (HSA) is covalently attached to 40-nm poly(chloromethyl)styrene-modified latex particles. This reagent, along with monoclonal antibody to HSA, is aliquoted into the aca reagent pack along with polyethylene glycol 8000 in a tablet form (giving a final reaction concentration of 15 g/L). A 150 mmol/L phosphate buffer, pH 7.8, is used to fill the reagent pack in the instrument and the agglutination reaction is monitored at 340 nm. The sample volume is 100 µL and the calibration curve covers the range 2–250 mg/L. Evaluation of commercial scale reagents against the Beckman Array nephelometric immunoassay system gave a Deming regression correlation of aca = 0.87 x Beckman + 8.5,r = 0.995, n = 145. Mean analytical recovery was 104 ± 4.5%, n = 20, and there was no evidence of a lack of parallelism. Interassay precision was 8.8% at 10.0 mg/L and <2.5% at >65 mg/L. Calibrator stability was in excess of 60 days. A small reference range study (24-h urine collections, n = 27) gave a mean of 5.6 mg/L with a range of 0.5–16.2 mg/L. Analytical sensitivity (2.5 SD from zero) was 0.40 mg/L.

Key Words: indexing terms: calibration stability • computer cooptimization

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