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.

Distressing somatic symptoms are ubiquitous both in mental disorders and medical diseases. From a psychometric perspective, the structure of somatic symptom distress is unclear, and little is known about the strengths of associations to related constructs, such as health anxiety and somatosensory amplification. To clarify the structure of somatic symptom distress and to explore associations to health anxiety, somatosensory amplification, and functional somatic syndromes, data sets of 2 samples of college students from Germany (N = 1,520) and Switzerland (N = 3,053) were investigated with confirmatory factor analysis with robust estimation. A bifactor model (with 1 general and 4 orthogonal specific symptom factors-gastrointestinal, fatigue, cardio-pulmonary, and pain symptoms) revealed the best model fit. Medium-sized associations were found among latent factors of general somatic symptom distress, health anxiety, and depression. First evidence for the construct validity of the latent variables within the proposed bifactor structure was gained by observing (a) strong associations between the general somatic symptom distress factor and somatosensory amplification and (b) significant associations between both the general somatic symptom factor as well as the symptom-specific factors with functional somatic syndromes. The results offer a theoretically and psychometrically plausible model for the structure of somatic symptom distress and suggest a distinction between cognitive-affective and sensory aspects of symptom perception. The findings are compatible with current cognitive psychological and neuropsychological approaches to symptom perception and imply that somatic symptom distress is a multidimensional phenomenon that is both strongly linked to but also clearly separable from related constructs.

Medically unexplained symptoms (MUS) are one of the key features of somatoform disorders. Although MUS are currently treated as both categorical (in terms of the diagnosis of somatoform disorders) and dimensional (in terms of quantitative measures of somatization/somatic symptom reporting), little is known about the empirical latent structure of MUS. Using taxometric analyses, the latent structure of somatic symptom reporting was analyzed with the Patient Health Questionnaire (PHQ)-15 in two student samples (N=782 and N=2,577) and a primary care sample (N=519). We applied three popular taxometric methods: Maximum Eigenvalue (MAXEIG), Mean Above Minus Below a Cut (MAMBAC) and Latent-Mode (L-Mode). Simulation data were created to evaluate the appropriateness of the data for our analyses and to create the comparison curve fit index (CCFI) as an objective outcome measure. The results of all taxometric methods in any of the three data sets were in favor of a dimensional solution (CCFI<.50). Simulated taxonic and dimensional datasets differed substantially and the samples were appropriate for taxometric analysis. Accordingly, the latent structure of somatization/somatic symptom reporting as assessed by the PHQ-15 is dimensional in both primary care and student samples. Implications regarding the practical application as well as models of etiology and pathogenesis of somatic symptom reporting are discussed.

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The findings suggest that IBS symptom severity might best be understood as a continuous and multidimensional construct which can be reliably and validly assessed with the B-IBS.