Comparative Measurement of Rheumatoid Factor in Serum and Synovial Fluid of Rheumatoid Arthritis Patients by ELISA and Latex-Agglutination Test

Rheumatoid factors (RF) are autoantibodies with specificity for the Fc portion of human IgG. Production of RF is a characteristic feature of rheumatoid arthritis (RA) and is detectable in high titer in about 90% of these patients. In this study, we measured total IgM, IgA, IgMRF and IgARF in serum and synovial fluid of 45 RA patients by ELISA and the results were compared with those obtained by latex agglutination test (LAT). The results show that from 45 RA patients, 93% (43:45) and 78% (35:45) were seropositive in ELISA and LAT, respectively. The sensitivity limit of LAT was approximately 10 µg/ml, whereas less than 10 ng/ml of RF was detectable by ELISA. There was a highly significant correlation between the concentration of IgMRF and latex agglutination titer in both serum (r = 0.78, p<0.0005) and synovial fluid (r = 0.66, p<0.05). Less significant correlations were observed for IgARF in serum (r = 0.65, p<0.05). While the concentration of total IgM and IgA were significantly higher in serum compared to synovial fluid, this however, was not reflected in the IgMRF or IgARF titer. In summary, our findings suggest that employment of isotype specific ELISA is inevitable and necessary for the quantitative measurement of RF isotypes other than IgM, which may be of clinical importance for diagnosis of disease severity and extra-articular complications of RA.