Longitudinal evaluation of the psychometric stability of the somatic symptom scale-8 (SSS-8) in a large German general population sample

The Somatic Symptom Scale (SSS-8) is a brief 8-item tool for screening common somatic symptoms. This study is the first to examine its temporal invariance and stability across three COVID-19 pandemic time points, while also analyzing changes in SSS-8 scores in the general population and across groups (sex, age, relationship status, poverty status), and testing temporal stability and measurement invariance. Longitudinal data from three pandemic time points in a large population-based sample were used. Temporal stability was assessed using Pearson correlations, and confirmatory factor analysis with measurement invariance testing examined factor validity and temporal invariance. The SSS-8 decreased from the first to the second measurement and subsequently increased between the second and third time points. The scale demonstrated good internal consistency (α ≥ 0.78, ω ≥ 0.82) and temporal stability, with test-retest coefficients ranging from 0.696 to 0.752. Across groups, female sex, age under 60, being not at-risk-of-poverty, and no necessary medical visits during the pandemic were consistently associated with lower test-retest coefficients than their counterparts. Confirmatory factor analysis showed good model fit (CFI = 0.985, TLI = 0.975, RMSEA = 0.040, SRMR = 0.035), and measurement invariance across time points was established (ΔCFI < 0.010). Consistent with other studies, somatic symptom burdening initially decreased and subsequently increased at the third measurement time point. The increase is discussed in the context of the pandemic and the serious political crisis in 2022. Overall, the scale demonstrated good psychometric properties including sensitivity to major crises of life conditions.