Clinical Phenotype, Treatment Exposure, Treat-to-Target Status, and Longitudinal Outcomes in Juvenile-Onset Systemic Lupus Erythematosus: A Single-Center Retrospective Cohort Study

Background/Objectives: Treat-to-target assessment is central to the management of juvenile-onset systemic lupus erythematosus (jSLE). We primarily evaluated last-visit childhood lupus low disease activity state (cLLDAS) status and its limiting components, and secondarily described clinical phenotype, treatment exposure, and outcomes. Methods: This retrospective single-center cohort included 54 patients with jSLE followed during 2020–2025; one patient who died 10 days after diagnosis was excluded from last-visit analyses, and the remaining 53 had at least 6 months of follow-up. The Paediatric Rheumatology European Society (PReS)-endorsed cLLDAS definition was applied at the last visit using a retrospectively assigned physician global assessment (PGA). Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) and laboratory values at diagnosis and the latest assessment were compared descriptively. Results: Median age at diagnosis was 15.0 years, and median follow-up was 34.6 months (IQR 21.2–70.6). Biopsy-proven lupus nephritis occurred in 29.6% (class IV, 13/16). Median SLEDAI-2K was 8.0 at diagnosis and 4.0 at the latest assessment. At the last visit, 29/53 patients (54.7%; 95% CI 40.4–68.4) were in cLLDAS, and 32/53 (60.4%) met the SLEDAI-2K and glucocorticoid components; of 24 patients outside the target, 17 had SLEDAI-2K > 4 and 7 exceeded the glucocorticoid ceiling (both in 3). Conservative minimum rates of flare, infection-related hospitalization, and intensive care unit admission were 5.6, 5.2, and 1.4 per 100 patient-years, respectively; one early death occurred. Conclusions: Just over half of the patients were in cLLDAS at the last visit, with residual disease activity as the main barrier, followed by glucocorticoid dose. These observational data reflect outcomes within the available follow-up period and cannot establish treatment effectiveness or long-term prognosis.