OBJECTIVE: IgA rheumatoid factor (RF) is associated with greater disease activity and radiological progression in rheumatoid arthritis (RA). We examined whether measuring IgA RF subclasses gives additional clinically relevant information. METHODS: Total IgA RF plus IgA1 RF and IgA2 RF subclasses were estimated by ELISA using rabbit IgG as antigen in 144 patients with established RA. Disease activity was assessed by Disease Activity Score and Health Assessment Questionnaire; the acute phase response was assessed by C-reactive protein levels and joint damage by the Larsen score. RESULTS: Fifty percent of patients had elevated total IgA RF, 60% elevated IgA1 RF, and 50% elevated IgA2 RF. There were significant correlations between total IgA RF and both IgA RF subclasses (p < 0.0001). Measures of disease activity, the acute phase response, and joint damage were all significantly higher (p < 0.001 in each case) in patients with elevated total IgA RF and both IgA RF subclasses compared with RF negative cases. Disease duration influenced the relationship of IgA RF subclasses to joint damage. The Larsen score was only significantly higher in late disease (duration 5 years or more) in both IgA1 RF and IgA2 RF positive patients. CONCLUSION: We found IgA RF positive patients have more aggressive RA than negative cases. Measuring IgA1 RF and IgA2 RF subclasses did not give more information about clinical status than measuring total IgA RF alone
