Comparing the Patient Health Questionnaire – 15 and the Somatic Symptom Scale – 8 as measures of somatic symptom burden

Abstract

Purpose The Patient Health Questionnaire – 15 (PHQ-15) and the Somatic Symptom Scale – 8 (SSS-8) are self-report measures which assess somatic symptom burden. The present study investigates whether the two measures are comparable in terms of their psychometric properties and estimates of symptom burden. Method Item characteristics, reliability, symptom severity and construct validity with regard to other relevant psychological, health-related quality of life and disability measures were compared for the PHQ-15m and the SSS-8 in 294 primary care patients who participated in a randomized comparative effectiveness trial targeting pain and mood symptoms. Results The reliabilities of the PHQ-15m and the SSS-8 were α = 0.66 and α = 0.72, respectively. Both measures were highly correlated (r = 0.79). All item characteristics were comparable and both instruments showed the same pattern of correlations with instruments measuring depression, anxiety, pain, quality of life and impairment (r = 0.25 to 0.53). A 1-point score increase (worsening of somatic symptoms) on either instrument resulted in a 3.7% to 3.9% increase in the number of disability days reported for the last four weeks. Using the same severity thresholds (5: low, 10: medium, 15: high), both measures identified nearly identical subgroups of patients with regard to health-related quality of life and disability. Conclusion The PHQ-15m and the SSS-8 are comparable measures in terms of reliability and validity and severity classifications. These findings are in line with previous results and support the use of the SSS-8 as a valuable and short alternative to the original PHQ-15 in settings with limited assessment time.