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

Area of Science:

• Rheumatology and Immunology
• Psychological Assessment
• Psychometrics

Background:

• Systemic Sclerosis (SSc) is a rare autoimmune disease causing fibrosis and immune dysfunction.
• Depression is a common psychological distress in SSc patients, impacting quality of life and treatment adherence.
• The SCL-Dep6, a six-item subscale of the SCL-90-R, assesses core depressive symptoms.

Purpose of the Study:

• To evaluate the structural validity and reliability of the SCL-Dep6 for depression assessment in Systemic Sclerosis (SSc) patients.
• To assess criterion-related validity using the WHO-5 questionnaire.
• To investigate potential Differential Item Functioning (DIF) based on demographic factors.

Main Methods:

• Rasch analysis was applied to data from 188 SSc outpatients to evaluate the SCL-Dep6's structural validity.
• Criterion-related validity was assessed using Receiver Operating Characteristic (ROC) analysis with WHO-5 scores.
• Analysis included checks for model fit, reliability (Person Separation Index), threshold ordering, and DIF.

Main Results:

• Rasch analysis confirmed the unidimensional structure of the SCL-Dep6, with data fitting the Rasch model (χ² = 20.59, p = 0.056).
• Person Separation Index (PSI = 0.651) indicated borderline reliability; two items exhibited disordered thresholds.
• Criterion-related validity was supported (AUC = 0.7891), but DIF was noted for two items concerning sex and education.

Conclusions:

• The SCL-Dep6 demonstrates structural validity for identifying depression in Systemic Sclerosis (SSc) patients.
• Its brevity makes it a potentially useful tool for clinical settings.
• Further research is needed to improve sensitivity for mild symptoms and ensure equitable performance across different subgroups.