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Objectives SLE Risk Probability Index (SLERPI) is a machine-learning derived model to assist the diagnosis of SLE. This study aims to validate SLERPI in an independent cohort of patients with SLE for early diagnosis and disease prognostication.Methods This is a retrospective study of patients with SLE at Universitätsklinikum Erlangen. Patients with physician-diagnosed SLE between 1990-2000 were included. Clinical and serological parameters at diagnosis were analysed for the fulfilment of the 2019 European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria and the calculation of SLERPI based on its specified fourteen domains. Disease flare (defined by the Safety of Estrogen in Lupus Erythematosus -SLE Disease Activity Index Flare Index) within three years of disease diagnosis were retrieved.Results A total of 280 patients (236 females [84.3%], median diagnosis age of 31 [IQR 23–45] years) were included. The EULAR/ACR criteria were fulfilled in 240 (85.7%) patients at diagnosis. Hypocomplementemia and immunologic disorders (anti-dsDNA and/or anti-Sm and/or anti-phospholipid antibodies) were observed in 171 (61.1%) and 216 (77.1%) patients, respectively. The most frequently fulfilled SLERPI clinical features were arthritis (143 patients, 51.1%), subacute cutaneous lupus erythematosus/discoid lupus erythematosus (93 patients, 33.2%), and malar rash/maculopapular rash (84 patients, 30.0%). Overall, the median SLERPI score was 10.8 [IQR 8.1–14]. Among 239 (85.4%) patients with total SLERPI scores above the cut-off of 7, 228 also fulfilled the EULAR/ACR criteria.Among 258 patients with complete medical records within three years of diagnosis, 115 (44.6%) and 82 (31.8%) patients experienced disease flare (any) and severe disease flare, respectively. Patients with disease flare (any) within three years had higher baseline SLERPI score at diagnosis (9.0 [7.5-12.5] vs 11.0 [9.0-14.5], p=0.010).Conclusions SLERPI demonstrated robust diagnostic performance in an independent cohort of patients with SLE. A higher baseline SLERPI score was significantly associated with an increased risk of subsequent disease flares within three years of diagnosis.