Head-to-head Comparison of Somatic Symptom Scales: The Patient Health Questionnaire (PHQ-15) and the Somatization Scale of the Four-Dimensional Symptom Questionnaire (4DSQ-S)

Objective: The goal of this study was to compare the 15-item Patient Health Questionnaire (PHQ-15) and the somatization subscale of the Four-Dimensional Symptoms Questionnaire (4DSQ-S) with respect to their latent structure and reliability, and to examine whether their scores are affected by age and gender, and whether the scales measure the same construct(s).

Methods: The study population consisted of individuals with a tendency to experience persistent somatic symptoms, recruited in multiple healthcare settings, who completed the PHQ-15 and 4DSQ-S concurrently. We analyzed the scales' latent factor structure using confirmatory factor analysis (CFA), the scales' reliability, and differential item functioning (DIF) due to age and gender. We performed a head-to-head comparison by fitting structural equation models of the questionnaires' factors.

Results: We included 234 participants. CFA showed that both questionnaires fitted a bifactor model with a general factor and four specific factors, three of which (labeled "musculoskeletal", "gastrointestinal", and "cardiopulmonary") were substantively similar. Both scales were essentially unidimensional. The reliability of the PHQ-15 and 4DSQ-S was equally high (omega 0.933 and 0.942, respectively). DIF-analysis showed minor DIF for age in one item of each questionnaire, with negligible impact on the scale score. Head-to-head comparison showed that the PHQ-15 and 4DSQ-S measured the same constructs. We present PHQ-15 - 4DSQ-S cross-walk tables.

Conclusions: Both questionnaires mainly measure a single somatic symptom burden dimension of which all symptoms (covered by the questionnaires) are adequate indicators. They do so equally accurately and they behave the same across gender and age categories.