Assessing the structural validity of the SCL-Dep6 depression subscale using rasch analysis

Systemic Sclerosis (SSc) is a rare and complex autoimmune disease characterized by skin and internal organ fibrosis, vascular abnormalities, and immune dysfunction. In addition to physical symptoms, many patients experience psychological distress, particularly depression, which can negatively impact treatment adherence and quality of life. The SCL-Dep6 is a subscale of the SCL-90-R questionnaire, consisting of six items that assess core aspects of depression, including low mood, anhedonia, guilt, fatigue, and suicidal thoughts. The aim of this study was to evaluate the structural validity of the SCL-Dep6 in patients with SSc using Rasch analysis. Criterion related validity was assessed using WHO-5. The sample included 219 outpatients with SSc recruited at the Careggi University Hospital in Florence, Italy. Rasch analysis was performed on 188 participants and showed that the data fit the model (χ² = 20.59, p = 0.056), confirming the unidimensional structure of the scale. The Person Separation Index (PSI = 0.651) indicated borderline reliability in this sample. Two items showed disordered thresholds. Differential Item Functioning (DIF) related to sex and education was observed for two items. Criterion related validity was supported by ROC analysis, with an area under the curve (AUC) of 0.7891. In brief, the SCL-Dep6 showed to be valid for identification of depression in patients with SSc. Its brevity makes it suitable for clinical use, although further research is needed to improve sensitivity to mild symptoms and ensure fairness across subgroups.