The effect of reaction temperature for nephelometric assays for rheumatoid factor

Even using the same assay parameter, reagent and calibrator (N-latex RF kit II), the results of the assay for serum rheumatoid factors (RFs) with the Behring Nephelometer Analyzer (BNA) were higher than those with the Behring Nephelometer II Analyzer (BNII) ([BNII]=0.76 [BNA]-5.7 kIU/l, r=0.997, Sy/x=60.73, n=99). The mean bias (BNA minus BNII)+/-S.D. was 52.7+/-85.5 using the Bland and Altman plot method, and the bias was not constant. The only difference in the assay condition with the two methods was the reaction temperature with the BNA being performed at room temperature (25+/-1 degrees C) and the BNII being performed at 37 degrees C. The ratio of the results with the BNII to the BNA (BNII/BNA) ranged from 0.23 to 1.18. A significant difference was observed in the BNII/BNA ratio in patients with high levels of C-reactive protein (CRP) over 2.0 mg/l (mean BNII/BNA ratio; 0.78) in comparison to patients with normal CRP levels under 2.0 mg/l (mean BNII/BNA ratio; 0.65) (P<0.01). The RF concentrations with the BNA were reduced by addition of urea, which has been used as a mild protein-denaturing agent, and there was a significant correlation between the values calculated as (1-value treated with urea/original value without urea)x100 and the BNII/BNA ratio (r=0.652, P<0.01). These data suggested that the bias between the RF values obtained by the BNA and BNII might be caused by the variation in the reactivity of autoantibodies, which might be decreased in some inflammatory diseases.