Assessment of myositis specific antibodies in primary chronic arthritis: single centre prospective study

Among the myositis specific antibodies (MSAs), the antisynthetase (anti-ARS) and the anti-MDA5 antibodies are those more frequently characterised by the occurrence of joint involvement. We aim to define the prevalence of MSAs in patients with established rheumatoid arthritis (RA), psoriatic arthritis (PsA), or undifferentiated polyarthritis (UPA).

From January 2021 to December 2024, all RA, PsA and UPA patients prospectively followed in our Early Arthritis Clinic (EAC), were evaluated. Changes in diagnosis, clinical/laboratory signs of muscle/lung/skin involvement at onset or during the follow-up, overlap syndromes, anti-ENA or cytoplasmic ANA positivity, Raynaud's phenomenon, and less than 24 months of follow-up were exclusion criteria. Baseline serum samples were tested for MSAs (line-blot). Positivity was defined according to manufacturers' instructions.

143 patients were enrolled (93 females, 65%; 67 AR, 47%; 50 UPA, 35%, 26 PsA, 18%). Line-blot resulted positive in 10 (7%), weak-positive in 12 (8%), and borderline in 26 cases (18%). The remaining 95 patients (67%) were negative. MSAs positivity was anti-cN1A in 3 cases and anti-ARS and anti-MDA in 4 cases each. Weak positivity was found for anti-ARS (4), and anti-PM-Scl75 (3). Borderline results showed a high number of anti-ARS and aMDA5 (12, 46%). No variables were associated with MSA positivity.

MSAs positivity may be observed in one third of patients with primary isolated arthritis. About half of these cases displayed full or weak positivity for MSAs, whereas the remaining half displayed borderline results. Clinicians should be aware that MSA should be assessed only in case of effective clinical need.