Comparison of Rheumatoid Factor Testing Methods: Nephelometric Assay Versus Latex Agglutination Assay

Rheumatoid factor (RF) is one of the criteria used for diagnosis of rheumatoid arthritis (RA). The method that has been used in our laboratory service for many years is latex agglutination assay that gives semi-quantitative results. We are going to change from this method to nephelometry that gives continuous results. The cut-off point of RF by nephelometry, comparison of these 2 methods and the 4 supplying companies were determined. Serum samples were collected from 70 patients with RA, 22 patients with various collagen diseases and 150 blood donors or normal old people. RF values by nephelometry and 4 commercial latex agglutination assays, that were latex Alexon, Shield diagnostics, Biosystem, and Behring diagnostics, were determined and compared. The results showed that the cut-off point of RF by nephelometry was 14 IU/ml and the sensitivity and the specificity was 81% and 95% respectively. The sensitivity and the specificity of latex agglutination assays by 4 companies were 76% and 94%, 68% and 97%, 62% and 98% and 64% and 98% respectively. We concluded that nephelometry gave higher sensitivity than latex agglutination assays and Latex Alexon had the highest sensitivity when comparing among the 4 companies