Background Systemic Lupus Erythematosus (SLE) is a complex disease which can potentially involve any organ and, therefore, has a wide range of clinical manifestations (i.e., subphenotypes) .
Objectives To propose a novel methodology for building an index of severity in patients with SLE.
Methods A cross-sectional analytical study was done on 319 SLE patients (1997 ACR). A structural model for the latent trait in the 2pl item response theory model was fitted following the Ogasawara's formulation  in order to characterize a severity construct of 17 variables. In addition, the model analyzed possible associations of the severity latent trait with other patient covariates including cardiovascular disease, age at onset of the disease, and expositional factors.
Results Baseline characteristics of patients were as follows: female gender 91%, median age 37 (IQR 21) years, median duration of the disease 5 (IQR 10) years, lupus nephritis 37.3%, and central nervous system involvement in 16%. Several symptoms were excluded from the severity construct due to inconsistency according to Cronbach-Mesbah Curve including lupus nephritis, diffuse alveolar hemorrhage, and vasculitis. A total of 11 symptoms were included in the item response theory model showing three levels of disease severity (Figure 1). The only covariate registered that reaches an association with severity was coffee consumption.
Conclusions Symptoms excluded due to inconsistency share the same pathogenic mechanism in SLE, which corresponds to autoantibody-driven tissue damage, and are associated with high mortality risk. The remaining symptoms are related to inflammation and vasculopathy, and in most cases, require a less aggressive immunosuppressive treatment . The order of symptoms on the present scale of severity coincides with clinical experience, and the three levels of performance can be associated with low, moderate, and high severity. The present index of severity is useful for SLE patients with less severe phenotype (i.e., without nephritis, diffuse alveolar hemorrhage, and vasculitis). A further replication study is necessary to validate these results.
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Disclosure of Interest None declared