Background: Rheumatoid factor (RF) is an IgM antibody against the Fc portion of IgG, which together form an immune complex. RF is an important criterion in the diagnosis of early-stage rheumatoid arthritis (RA) and prognosis of RA pathogenesis, as higher levels of RF indicate a higher possibility of more damage. Although 2/3 to 3/4 of patients that undergo ordinary standard tests and have final clinical diagnosis are also positive for RF, a 70-90% prevalence of RF among RA patients can be achieved, depending on the method of detection and the target antibody, IgG or IgM. In this study, we measured the frequency of IgG and IgM RF isotypes using the ELISA and latex agglutination methods and compare these results with those of a hospital control group, tested using standard methods, in order to determine the best method for the measurement of RF.

Methods: Of the patients referred to the Rheumatology Clinic of Imam Khomeini Hospital during 2005-2006, one hundred randomly selected rheumatoid arthritis patients, 75 females and 25 males, with classical or definite rheumatoid arthritis (defined by the criteria of the American College of Rheumatology), with a short disease duration of 12-24 months, underwent testing for RF using the latex method for IgM and ELISA for IgM-IgG. The healthy control group (75 females and 25 males) were tested for RF using the ELISA method for IgM-IgG. The variables were compared using the Pearson's chi-square test.

Results: We found that the measurement of RF among RA patients using did not differ significantly between the two methods. The immune complex in RA is mainly IgM. The positive IgM results in RF patients using two similar methods showed a significant relationship by Pearson's correlation co-efficient (r=0.60, p<0.001). In addition, comparison of the IgM and IgG RF by ELISA showed a weak correlation with low significance (r=0.10, p<0.001). In sum, this study showed a significant difference (r=0.24, p<0.001) between the IgM in RA patients and that in healthy people, who had no IgM or IgG RF.

Conclusion: Approximately 75% of confirmed RA cases had the IgM RF; however, we found little advantage in using the one method over the other, nor was the measurement of IgG more useful than IgM as a diagnostic criteria.