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

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.