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Determination of C-reactive protein: comparison of three high-sensitivity immunoassays.

Atherosclerosis is an inflammatory condition (1), and progression to atherothrombotic events is associated with systemic evidence of low-grade inflammation. Circulating concentrations of C-reactive protein (CRP) within the range previously considered to be normal have been shown to predict future cardiovascular events independently in initially healthy individuals as well as in patients with manifest atherosclerotic disease, even in the absence of hyperlipidemia (2)(3)(4). The detection of such subclinical inflammation requires high-sensitivity methods for CRP measurement. Recently, we reported on the analytical performance of a new sensitive solid-phase monoclonal-polyclonal IRMA (5). This assay has a very low analytical detection limit of 0.05 mg/L, and in a cohort study, CRP determined by this method showed a strong linear association with future coronary endpoints (4). Here we compare the performance of the IRMA specifically designed to evaluate the low CRP range with that of an ultrasensitive latex-enhanced immunonephelometric test and an immunoturbidimetric assay in a large case–control study. Additionally, we estimated the risk for coronary artery disease (CAD) given an increased CRP concentration as determined by the three assays.

The cases consisted of 312 patients 40–68 years of age with clinically stable CAD, who underwent elective coronary angiography in the Department of Cardiology at the University of Ulm Medical Centre and who had one or more epicardial coronary stenoses of ≥50% in luminal diameter. Voluntary blood donors (n = 476) from the local blood bank served as controls. In these individuals, symptomatic CAD was excluded by the Rose questionnaire. Venous blood was obtained under standardized conditions in the morning before diagnostic coronary angiography in cases and after blood donation in controls. EDTA-plasma specimens were centrifuged at 3000 g for 10 min, immediately aliquoted, and frozen at −70 °C until analysis. Cases and controls were “frequency matched” by age and gender, …