The Somatic Symptom Disorder - B Criteria Scale (SSD-12): Factorial structure, validity and population-based norms

PURPOSE: The Somatic Symptom Disorder - B Criteria Scale (SSD-12) assesses the psychological features of DSM-5 Somatic Symptom Disorder (SSD). The present study investigates the dimensionality and psychometric properties in a general population sample and provides norm values.

METHOD: Test dimensionality was evaluated via confirmatory factor analysis and nonparametric item response theory. Correlational analyses and logistic regression models based on related measures (SSS 8, PHQ-2, GAD-2, Health Care Utilization) were used to derive predictive validity. Age and gender specific norms were derived via quantile regression.

RESULTS: The SSD-12 has good item characteristics and excellent reliability (Cronbach's α=0.95). Confirmatory factor analyses revealed a high correlation between the three proposed psychological subscales interpreted as cognitive, affective and behavioral aspects, indicating a general factor model of the SSD-12 in the general population (n=2362, CFI=0.99, TLI=0.998, RMSEA=0.09, 90% CI: 0.09-0.1). SSD-12 total sum-score was significantly associated with somatic symptom burden (r=0.73, p<0.001), general anxiety (r=0.63, p<0.001) and depressive symptoms (r=0.64, p<0.001). Patients with a higher SSD-12 symptom burden reported higher general physical and mental health impairment and a significantly higher health care use.

CONCLUSION: The SSD-12 is a reliable and valid self-report measure of the psychological characteristics of DSM-5 Somatic Symptom Disorder. The provided norms enable researchers and clinicians to compare SSD-12 scores with reference values of a general population sample.