Prevalence and clinical correlates of rheumatoid factor and anticitrullinated protein antibodies in patients with idiopathic inflammatory myopathy

OBJECTIVE: As rheumatoid factor (RF) and anticitrullinated protein antibodies (ACPAs) are not routinely tested in idiopathic inflammatory myositis (IIM), little is known about their prevalence and clinical implications in this patient group. In antisynthetase syndrome (ASS), presence of ACPA is reportedly associated with more severe and erosive arthritis. We aim to retrospectively determine the prevalence of RF and ACPA in a cross-sectional cohort of 121 patients diagnosed with IIM and to assess clinical associations. METHODS: Serum samples from 121 patients diagnosed with polymyositis (n=30), dermatomyositis (n=41), ASS (n=37), inclusion body myositis (n=1), necrotising autoimmune myopathy (n=5) or overlap myositis (n=7) were analysed. RF was evaluated by nephelometry (Immage 800, Beckman–Coulter); anti-CCP antibodies were identified using fluoro enzyme immunoassays (Immuno-Cap 250, Thermo Fisher). Values above 40 IU/mL and 7 U/mL were considered positive for RF and ACPA, respectively. RESULTS: The prevalence of RF and ACPA was 9.09% and 4.96%, respectively. No significant differences were observed between RF/ACPA positive versus negative patients. There was a numerical trend for RF-positive IIM patients to be older and have lower forced expiratory volume in 1 s levels. CONCLUSIONS: RF and ACPA are prevalent in IIM, although we detected a lower prevalence than reported in previous studies. Presence of these antibodies in patients with IIM patients is not clinically relevant in our cohort.