Background: Medically unexplained symptoms are the hallmark of somatoform disorders and functional somatic syndromes.
Purpose: Although medically unexplained symptoms represent a common phenomenon both in the general population as well as in medical settings, the exact latent structure of somatic symptoms remains largely unclear.
Method: We examined the latent structure of medically unexplained symptoms by means of the Patient Health Questionnaire-15 (PHQ-15) questionnaire (i.e., a popular symptom checklist) and provide support for the construct validity of our model. The data were analyzed using confirmatory factor analysis in a general population sample (study 1; N = 414) and in a sample of primary care patients (study 2; N = 308). We compared four different latent structure models of medically unexplained symptoms: a general factor model, a correlated group factor model, a hierarchical model, and a bifactor model.
Results: In study 1, a bifactor model with one general factor and four independent specific symptom factors (i.e., gastrointestinal, pain, fatigue, and cardiopulmonary symptoms) showed the best model fit. This bifactor model was confirmed in the primary care sample (study 2). Additionally, the model explained 59 % of the variance of the irritable bowel syndrome (IBS). In this structural equation model, both the general factor (14 %) as well as the gastrointestinal symptom factor (42 %) significantly predicted the IBS.
Conclusion: The findings of both studies help to clarify the latent structure of somatic symptoms in the PHQ-15. The bifactor model outperformed alternative models and demonstrated external validity in predicting IBS.