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Objectives: The aim of this study is to investigate the effect of anti-interleukin (IL)-1/-6 biologics on systemic juvenile idiopathic arthritis (sJIA)-associated macrophage activation syndrome (MAS)., Methods: Demographic, clinical and laboratory data of patients followed up with a diagnosis of sJIA-associated MAS assessed from sixteen paediatric rheumatology centres across the country. The clinical and laboratory features of MAS developing while on biological drugs were compared with those without this treatment., Results: One hundred and sixty-two patients were included in the study. Forty-five of the MAS events were detected under the effect of anti-IL-1/-6 biologics, while the patients experiencing the remaining 155 events have not received biological treatment in the last three months. Platelet count [128 (72-232) vs 199 (130-371) 109/l], ferritin level on admission [1107 (676-2050) vs 2863 (1193-9562) ng/ml], C-reactive protein level [15.4 (2.9-56) vs 90 (32-160) mg/l], erythrocyte sedimentation rate [13 (3-36) vs 43.5 (13-77) mm/h] and fever duration [5 (4-7.5) vs 10 (7-14.3) days] were found lower in the group under the impact of anti-IL-1/-6 biologics. Among patients treated with biologics, 26.6% did not meet the published 2016 MAS classification criteria at presentation. The rates of hepatomegaly and splenomegaly were relatively lower in the canakinumab-treated group when compared with those receiving other biologicals or to patients, not on biologicals., Conclusion: Anti-IL-1/-6 therapies can mask the clinical and laboratory features of MAS, and proposed guidelines for MAS classification criteria may not be met., (© The Author(s) 2024. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Objective: Rheumatoid factor (RF)-positive polyarthritis is the least common type of juvenile idiopathic arthritis (JIA). Functional disability in RF-positive polyarthritis patients is much more severe than in patients with other subtypes; but data on this subtype alone is limited. This study aimed to analyze clinical features, long-term follow-up, treatment response, and remission status in a large multicenter cohort of RF-positive polyarthritis patients. Methods: This retrospective study included RF-positive polyarthritis patients that were followed up for ≥ 6 months between 2017 and 2022 by the Pediatric Rheumatology Academy (PeRA)-Research Group (RG). Data on patient demographics, clinical and laboratory characteristics were obtained from medical charts. JIA treatments and duration of treatment were also recorded. The patients were divided into 2 groups based on methotrexate (MTX) response, as follows: group 1: MTX responsive, group 2: MTX unresponsive. Clinical and laboratory findings were compared between the 2 groups. Results: The study included 56 (45 female and 11 male) patients. The median age at onset of RF-positive polyarthritis was 13.2 years [(interquartile range) (IQR): 9.0–15.0 years] and the median duration of follow-up was 41.5 months (IQR: 19.5–75.7 months). Symmetrical arthritis affecting the metacarpophalangeal and proximal interphalangeal joints of the hands was commonly observed. Subcutaneous MTX was the preferred initial treatment; however, it was ineffective in 39 (69.6%) of the patients. Of 25 patients followed for 24 months, 56% still had active disease at 24 months. Conclusion: During 2 years of treatment, 44% of RF-positive polyarthritis patients have inactive disease, and they should be considered as a distinct and important clinical entity requiring aggressive and early treatment. [ABSTRACT FROM AUTHOR]