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BACKGROUND: Historically, measures of symptom severity of irritable bowel syndrome with constipation (IBS-C) in clinical trials have not met the evidence requirements described in the FDA guidance on patient-reported outcomes (PROs), which describes the evidentiary requirements and review criteria for patient-reported outcome measures intended to support product approval or labelling claims. AIM: Data from two phase 3 trials (N = 1608) of linaclotide for the treatment of IBS-C were analysed to evaluate the psychometric properties of patient-reported outcome measures assessing changes in the severity of abdominal and bowel symptoms. METHODS: A set of patient-reported outcome assessments addressing abdominal and bowel symptoms, the IBS-C Symptom Severity Measures, were administered daily using interactive voice response system technology. Intraclass correlation coefficients (ICCs), Pearson correlations, factor analyses, F-tests and effect sizes were computed to evaluate the reliability, construct validity, discriminating ability and responsiveness of the IBS-C Symptom Severity Measures in a clinical trial context. RESULTS: The IBS-C Symptom Severity Measures showed highly satisfactory test-retest reliability (ICCs ranging from 0.79 to 0.95) and construct validity. Factor analyses indicated one factor for abdominal symptoms and another for bowel symptoms. Known-groups F-tests comparing subgroups based on various responder definitions were statistically significant and in the expected direction, substantiating the discriminating ability of the IBS-C Symptom Severity Measures. Responsiveness statistics (ranging from 0.6 to 2.1) demonstrated these measures are also capable of detecting change. CONCLUSIONS: The psychometric analysis results strongly support the reliability, construct validity, discriminating ability and responsiveness of the IBS-C Symptom Severity Measures and substantiate the conclusion of linaclotide treatment benefit
