Analytical Evaluation of Particle-Enhanced Immunonephelometric Assays for C-Reactive Protein, Serum Amyloid a and Mannose-Binding Protein in Human Serum

Abstract

Against a background of growing interest in more sensitive assays for quantifying various acute phase proteins, we evaluated the performance of recently developed tests for C-reactive protein (CRP), serum amyloid A (SAA) and mannose-binding protein (MBP) on the Behring nephelometer II (BN II). Sample results outside the calibration ranges of 3·5 to 220 mg/L for CRP, 3·3 to 215 mg/L for SAA and 0·09 to 5·6 mg/L for MBP were automatically re-measured at another dilution. The lower limits of detection were 0·01, 0·7 and 0·01 mg/L for CRP, SAA and MBP, respectively. The coefficients of variation (CV) for intra- ( n ⩾ 20) and inter- ( n ⩾ 15) assay precision were < 5·2% and < 8·5%, respectively, for the three proteins at concentrations representing low, normal and high. Linearity for each method was within 5% of the expected values throughout the calibration range. We observed no significant interference from bilirubin (up to 300 mg/L) or haemoglobin (up to 10 g/L) for the three tests. Method comparison studies performed for CRP and SAA yielded the following results: y (CRP on BN II) = 0·75 x (ELISA, Hemagen) −0·25 mg/L ( r = 0·981, S_y/x = 2·1 mg/L; y (SAA on BN II) = 1·44 x (ELISA, Hemagen) −9·9 mg/L ( r = 0·972, S_y/x = 6·9 mg/L), where ELISA is enzyme-linked immunosorbent assay. Reference intervals established in 261 adult blood donors (aged 36·2 ± 9·0 years) were found to be log-normal with 2·5th, 50th and 97·5th centiles of < 0·17, 100 and 10·1 mg/L for CRP, < 0·84, 2·10 and 9·70 mg/L for SAA; and 0·30, 1·28 and 4·10 mg/L for MBP. We observed no relationship with CRP concentration and age; however, SAA levels increased with age while MBP levels decreased. The BN II provides a simple, rapid and sensitive system for measuring CRP, SAA and MBP in human serum.