Objective: The Patient Health Questionnaire-15 (PHQ-15) is a frequently used questionnaire to assess somatic symptom burden. Recently, the Somatic Symptom Scale-8 (SSS-8) has been published as a short version of the PHQ-15. This study examines whether the instruments' psychometric properties and estimates of symptom burden are comparable.
Methods: Psychosomatic outpatients (N= 131) completed the PHQ-15, the SSS-8 and other questionnaires (PHQ-9, GAD-7, WI-7, SF-12). Item characteristics and measures of reliability, validity, and symptom severity were determined and compared.
Results: The reliabilities of the PHQ-15 and SSS-8 were α= 0.80 and α= 0.76, respectively and both scales were highly correlated (r= 0.83). The item characteristics were comparable. Both instruments showed the same pattern of correlations with measures of depression, anxiety, health anxiety and health-related quality of life (r= 0.32 to 0.61). On both scales a 1-point increase was associated with a 3% increase in health care use. The percentile distributions of the PHQ-15 and the SSS-8 were similar. Using the same thresholds for somatic symptom severity (5, 10, and 15 points), both instruments identified nearly identical subgroups of patients with respect to health related quality of life.
Conclusion: The PHQ-15 and the SSS-8 showed similar reliability and validity but the comparability of severity classifications needs further evaluation in other populations. Until then we recommend the use of the previously established thresholds. Overall, the SSS-8 performed well as a short version of the PHQ-15 which makes it preferable for assessment in time restricted settings.