Outcomes of undifferentiated peripheral inflammatory arthritis in real-world practice. A longitudinal cohort study

Objectives Undifferentiated peripheral inflammatory arthritis (UPIA) may have 3 different courses, including evolution to differentiated arthritis, remaining undifferentiated, and self-limited course. The purpose of this study was to provide a real-world evidence for predictors of outcomes in UPIA in a longitudinal cohort of patients. Methods Patients enrolled in the CTDRC-UA cohort were screened for eligibility. Inclusion criteria were: (i) having synovitis in ≥ 1 joint, (ii) not meeting the criteria of any other rheumatic disease, (iii) having at least 2 visits per year, iv) included in the cohort during the period of 2004 to 2021, and (v) having active disease at cohort entry. Two hundred and three patients who met the inclusion criteria were followed up until January 2023. Results Medication-free remissions occurred in 42 (20.7%) cases. In 24 (11.8%) cases, the disease met the criteria of other rheumatic diseases, of which rheumatoid arthritis (RA) was the most common. In addition, joint damage occurred in 33 (16.3%) cases. Predictors of medication-free remissions were absence of comorbidity, starting a sustained remission at ≤ 6 months, and having no flare. Factors associated with disease evolution to RA were anti-citrullinated peptide antibody (ACPA) positivity, non-adherence to therapy, not going into sustained remission and having flare. Delay in treatment for > 3 months and being ACPA positive were the predictors of joint damage. Conclusion Although the majority of UIPA cases treated with step-up combination therapy with DMARDs do not progress to RA, most require continued treatment and a few achieve medication-free remissions. Key Points • Undifferentiated peripheral inflammatory arthritis (UPIA) can progress to rheumatoid arthritis in 11% of cases; and lack of sustained remission, being anti-citrullinated peptide antibody positive, non-adherence to therapy, and having flare are its predictors . • Medication-free remissions occur in 21% of patients with UPIA; and absence of comorbidity, starting a sustained remission at ≤ 6 months, and having no flare are its predictors . • Initiating treatment in the window of opportunity may lead to a better joint outcome .