Evaluation of a quantitative immunoturbidimetric assay for rheumatoid factors.

A quantitative immunoturbidimetric assay for rheumatoid factors (RF) is described, based on the immunoprecipitation between aggregated human IgG and rheumatoid factors in serum. The resulting turbidity is measured photometrically at 340 nm. The method is standardized against the WHO international reference preparation and the results are expressed in international units per milliliter (int. units/mL). Results correlate well with those by different latex-agglutination techniques (r = 0.80-0.96). The correlation with Waaler-Rose test modifications were 0.75 and 0.92. The within-run and between-run coefficients of variations were respectively from 1.2 to 2.6% and 1.3 to 1.9% for high and low RF concentrations. Quantitative and reproducible results, together with high throughput of samples and compatibility with most clinical chemistry analyzers and photometers, make this new assay well suited for routine screening and monitoring of rheumatoid factors.