Irritable Bowel Syndrome (IBS) is a gastrointestinal disorder that affects up to 16% of the population. Thirty percent of IBS patients have a diagnosed psychiatric disorder, and 56% display features of mental illness. However, the inverse—the prevalence of IBS among psychiatric populations—has not been calculated on a large scale prior to our study. Using a shortened version of the Rome IV diagnostic questionnaire, we found that 27% of people with anxiety, depression, OCD, or PTSD also met diagnostic criteria for IBS. Major depressive disorder and panic disorder showed especially high IBS prevalence rates, and thirty percent of people with more than one mental disorder met IBS criteria.

Besides the high prevalence of IBS in psychiatric populations, another important finding in our study was that if the Rome III diagnostic criteria was used instead of the Rome IV, 51% of participants would have met criteria for an IBS diagnosis. This suggests that there may be patients with significant gastrointestinal distress who are not being diagnosed due to the more stringent Rome IV criteria.