A new method of measuring C-reactive protein, with a low limit of detection, suitable for risk assessment of coronary heart disease

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Abstract
A new indication has been proposed for C-reactive protein (CRP) as a prognostic risk marker of coronary heart disease (CHD). The new indication calls for accurate (true and precise) measurement of CRP within the conventional reference range (<5 mg/L). The existing turbidimetric and nephelometric methods do not cover the required measuring range and thus time-consuming and labour-intensive enzyme immunoassays have been used for the clinical studies focusing on CHD risk. We developed a new method based on micro-particle enhanced turbidimetry, which attained the required limit of detection, while keeping the upper measuring limit comparable to the existing turbidimetric and nephelometric methods. The superior characteristics of the new method were realised by mixing two types of microparticle reagents differing in microparticle size and reactivity of coated antibody. The analytical detection limit of the method was 0.28 mg/L with use of only 2.5 μ1 serum. The method showed good precision at 2 to 3 mg/L, the critical concentration range for CHD risk assessment. Other performance data including dilution linearity, method comparison, and interference study also met the requirements for the practical use in the clinical laboratories. Sera from 354 apparently healthy blood donors were measured in a reference range study. The reference range estimated after log-transformation was 0.16 mg/L to 7.57 mg/L CRP, with a total range of 0.09 mg/L to 21.0 mg/L. The distribution of CRP concentrations in this population was comparable to other results that established the use of CRP as a risk marker of CHD in a prospective study.