Measurement properties of the Patient Health Questionnaire 15 (PHQ-15) and Somatic Symptom Disorder B-criteria scale (SSD-12), including revised 1-week versions

The Patient Health Questionnaire 15 (PHQ-15), which measures somatic symptom burden, and the Somatic Symptom Disorder B-criteria scale 12 (SSD-12), which measures symptom preoccupation, are widely used questionnaires in behavioral medicine. This study built on the existing literature by evaluating the PHQ-15 and SSD-12 in Swedish, and in revised 1-week versions that could facilitate repeated measurements. The questionnaires were completed online by clinical trial participants with persistent physical symptoms (n = 194), and healthy volunteers (n = 160). For both conventional and revised 1-week versions, we evaluated item distributions, factor structure, internal consistency, construct validity based on correlations with other constructs, and test-retest reliability. Regardless of timeframe, the PHQ-15 factor structure combined general and domain-specific factors. The SSD-12 showed a three-factor structure reflecting Expectation of a chronic course, Health anxiety, and Symptom focus and impairment. For both questionnaires, internal consistency and construct validity were mostly supported, but adequate test-retest reliability was only observed in the clinical data and for averages of timepoints. Test-retest reliability in the healthy volunteer data was poor. This study is cautiously supportive of wider use of revised PHQ-15 and SSS-12 versions with a 1-week focus to facilitate repeated measurements of somatic symptom burden and symptom preoccupation in clinical populations.