An improved automated immunoassay for C-reactive protein on the Dimension (R) clinical chemistry system

Recent clinical data indicate that the measurement of the concentration of C-reactive protein ( CRP) requires a higher sensitivity and wider dynamic r ange than most of the current methods can offer. Our goal was to develop a totally automated and highly sensitive CRP assay with an extended range on the Dimension(R) clinical chemistry system based on particle-enhanced turbi dimetric-immunoassay ( PETIA) technology. The improved method was optimized and compared to the Binding Site's radial immunodiffusion assay using dise ase state specimens to minimize interference. Assay performance was assesse d on the Dimension(R) system in a 12-instrument inter-laboratory comparison study. A split-sample comparison (n = 622) was performed between the impro ved CRP method on the Dimension(R) system and the N Latex CRP mono method o n the Behring Nephelometer, using a number of reagent and calibrator lots o n multiple instruments. The method was also referenced to the standard mate rial, CRM470, provided by the International Federation of Clinical Chemistr y ( IFCC). The improved CRP method was linear to 265.1 mg/l with a detectio n limit between 0.2 and 0.5 mg/l. The method detects antigen excess from th e upper assay limit to 2000 mg/l, thereby allowing users to retest the samp le with dilution. Calibration was stable for 60 days. The within-run reprod ucibility ( CV) was less than 5.1% and total reproducibility ranged from 1. 1 to 6.7% between 3.3 and 265.4 mg/l CRP. Linear regression analysis of the results on the improved Dimension(R) method ( DM) versus the Behring Nephe lometer ( BN) yielded the following equation: DM = 0.99 x BN + 0.37; r = 0. 992. Minimal interference was observed from sera of patients with elevated IgM, IgG and IgA. The recovery of the IFCC standard was within 100 +/- 7% a cross multiple lots of reagent and calibrator. The improved CRP method prov ided a sensitive, accurate and rapid approach to quantify CRP in serum and plasma on the Dimension(R) clinical chemistry system. The ability to detect antigen excess eliminated reporting falsely low results caused by the 'pro zone effect'.