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

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.

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.

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.

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.