What Is Unspecified Functional Bowel Disorder? A Commonly Seen, Rarely Recognized, and Poorly Understood Diagnosis

INTRODUCTION: Unspecified functional bowel disorder (FBD-U) is a Rome IV diagnosis of exclusion and occurs when patients report bowel symptoms but do not meet the criteria for other functional bowel disorders (FBD; irritable bowel syndrome [IBS], functional constipation [FC], functional diarrhea [FDr], or functional bloating). Previous research suggests that FBD-U is as/more common than IBS. METHODS: One thousand five hundred one patients at a single-center tertiary care center completed an electronic survey. Study questionnaires included Rome IV Diagnostic Questionnaires, anxiety, depression, sleep, health care utilization, and bowel symptom severity measures. RESULTS: Eight hundred thirteen patients met Rome IV criteria for a FBD and 194 patients (13.1%) met criteria for FBD-U, representing the second most common FBD after IBS. Severity of abdominal pain, constipation, and diarrhea were lower in FBD-U compared with other FBD, whereas health care utilization was similar among the groups. Scores on measures of anxiety, depression, and sleep disturbances were similar in FBD-U compared with FC and FDr but were less severe than IBS. Between 25% and 50% of patients with FBD-U did not meet Rome IV criteria for other FBD due to timing of onset of the target symptom (e.g., constipation for FC, diarrhea for FDr, and abdominal pain for IBS). DISCUSSION: FBD-U, by Rome IV criteria, is highly prevalent in clinical settings. These patients are not represented in mechanistic studies or clinical trials for not having met Rome IV criteria for other FBD. Making future Rome criteria less stringent would minimize the number fulfilling criteria for FBD-U and maximize the true representation of FBD in clinical trials.