The Brief Symptom Inventory in the Swiss general population: Presentation of norm scores and predictors of psychological distress.

Psychological distress is an important and frequent health problem. The Brief Symptom Inventory (BSI) allows screening for psychological distress in clinical, general and research populations. We aimed to provide normative data for the BSI and the BSI-18 for the Swiss general population: We 1) present psychometric properties, 2) develop a Swiss T-standardization and validate it using a clinical sample, 3) describe psychological distress in the Swiss general population and the clinical sample, and 4) compare the means and T-standardized scores of the Swiss general population to different German norm populations. Using a cross-sectional study design, we invited a representative sample of the Swiss general population aged 18-75 years to the study. A sample of psychotherapy outpatients had competed the BSI before start of their therapy. We calculated scores for the nine scales of the BSI (three of them constitute the BSI-18), the T-standardization and the following BSI indices: Global Severity Index (GSI), Positive Symptom Total (PST), Positive Symptom Distress Index (PSDI), and Caseness (reaching T≥63 on the GSI or T≥63 on at least two of the scales). A total of 1238 general population participants completed the BSI (41.8% male; mean age 48.9 years). The BSI had good psychometric properties. The Swiss T-standardization showed good validity when applied in the clinical sample. Females reached a significantly higher GSI score than males (p<0.001). Older participants (p = 0.026), those with higher education (p <0.001), and those employed or retired (p<0.001) reached lower scores than participants aged 18-25 years, those with compulsory schooling, and unemployed participants, respectively. A total of 18.1% (CI: 16.0-20.5) participants of the general population and 75.2% (CI: 73.7-76.7) of the psychotherapy patients were considered cases with psychological distress. Our study presents detailed normative data for the BSI and the BSI-18 based on a representative sample of the Swiss general population. This information will be helpful for clinical applications and research in the Swiss and international context.