Development and validation of new disease-specific measures of somatization and comorbidity in IBS

Objective: To create and validate empirically derived questionnaires that measure non-gastrointestinal symptoms and disorders that co-exist with irritable bowel syndrome (IBS).

Methods: A systematic review of the world literature identified all non-GI symptoms and diagnoses known to have excess frequency in IBS patients. These data were used to create the Recent Physical Symptoms Questionnaire (RPSQ), which measures somatization (the psychological tendency to report multiple physical symptoms), and the Comorbid Medical Conditions Questionnaire (CMCQ). The psychometric properties of these questionnaires were assessed in two studies: 109 IBS patients in Study I; 286 IBS patients and 67 healthy controls in Study II.

Results: In Study I, the RPSQ and CMCQ showed high test-retest reliability (r=.88 and .95) and good internal consistency (Cronbach alphas: .86 and .70, respectively). In Study II, principal components analysis demonstrated that the RPSQ is a homogeneous somatization scale, but the CMCQ could be divided into 4 subscales: one for psychiatric disorders and 3 for different types of somatic disorders. Concurrent validity of the RPSQ was shown by strong correlations with the Cornell Medical Index (CMI) and the Brief Symptom Inventory-18 (BSI-18) somatization scales. The validity of CMCQ responses was not assessed. Discriminant validity was modest: the BSI-18 anxiety and depression scales were less strongly correlated with the RPSQ than the BSI-18 somatization scale. The RPSQ and CMCQ scores of IBS patients were significantly higher than the scores of healthy controls (P<.001).

Conclusions: The RPSQ and CMCQ are psychometrically sound measures of somatization and medical comorbidities in IBS.