Aim: To investigate the latent structure of an irritable bowel syndrome (IBS) symptom severity scale in a population of healthy adults.
Methods: The Birmingham IBS symptom questionnaire which consists of three symptom specific scales (diarrhea, constipation, pain) was evaluated by means of structural equation modeling. We compared the original 3-factor solution to a general factor model and a bifactor solution in a large internet sample of college students (n = 875). Statistical comparisons of competing models were conducted by means of χ (2) difference tests. Regarding the evaluation of model fit, we examined the comparative fit index (CFI) and the Root Mean Square Error of Approximation (RMSEA).
Results: Results clearly favored a bifactor model of IBS symptom severity (CFI = 0.99, RMSEA = 0.05) which consisted of a strong general IBS somatization factor and three symptom specific factors (diarrhea, constipation, pain) based on the subscales of the Birmingham IBS symptom questionnaire. The fit indices of the competing one factor model (CFI = 0.85, RMSEA = 0.17) and three factor model (CFI = 0.97, RMSEA = 0.08) were clearly inferior. χ (2) difference tests showed that the differences between the models were indeed significant in favor of the bifactor model (P < 0.001). Correlations of the four latent factors with measures of pain sensitivity, somatoform dissociation, fatigue severity, and demographic variables support the validity of our bifactor model of IBS specific symptom severity.
Conclusion: 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.
Keywords: Bifactor; Birmingham irritable bowel syndrome scale; Gastrointestinal; Irritable bowel syndrome; Rome-III criteria.