Automated Turbidimetry of Rheumatoid Factor without Heat Inactivation of Serum

Summary: We describe an improved turbidimetric procedure for rheumatoid factor measurement in human serum. Heat-aggregated human IgG is used äs antigen. Interference produced by the complement component Clq, which required a previous heat pretreatment of sera, is avoided by the addition of the chemical inhibitor, poly(vinyl sulphonic acid). This inhibitor eliminates Clq interference without affecting the attachment of rheumatoid factor to the Fc pari of IgG, thus permitting füll automation of the assay. After studying the reactivity of different heat-aggregated IgG preparations, we optimized the procedure for preparing the antigen in order to attain maximal reactivity: IgG was heated at 63 °C until the absorbance at 340 nm of a 1/10 dilution of the antigen lay between 0.65 and 0.95 absorbance units. The study of antigen stability showed a maximum aggregation two weeks after preparation. The antigen was then stable and could be üsed in the assay. The Standard curve extends from 30 to 500 l O3 IU/1. Intra- and inter-assay CV were less than 5.5%. Relative analytical sensitivity and specificity were 98.3 % and 95.2%, respectively. The results agreed well with those obtained by the non-improved turbidimetric procedure (r = 0.996) and with nephelometry values on the Behring nephelometer analyser (r = 0.964). The correlation with an enzyme immunoassay was also good (r = 0.923).. To initiate activation of the classical pathway of com