The diagnosis of irritable bowel syndrome (IBS) is one of exclusion of organic disease. Diagnosis should be based on patient history, physical examination, symptoms, and laboratory tests. Symptoms should have been present for at least 12 weeks, such that acute onset of pain, diarrhea, or any other symptoms should initially be presumed to be organic and not IBS until sufficient testing or history is obtained to rule out other causes. The Rome criteria are useful for differentiating irritable bowel syndrome (IBS) patients from others presenting with chronic abdominal pain. However, there are variants in presentation unrecognized by these criteria. They do not consider predominant bowel dysfunction, the presence of postprandial symptoms, or patients who do not have pain but have postprandial urge. The ACG panel recommends evaluation of abnormal stool frequency, form and passage, presence of bloating, and passage of mucus to help the clinician diagnose IBS.
To further differentiate irritable bowel syndrome (IBS) from organic causes, the patient history should include detailed information regarding recent weight loss, fever, frank or occult blood in stool, and pain or diarrhea severe enough to wake the patient from sleep. A detailed medication history should also be obtained to rule out gastrointestinal (GI) side effects as causal. Physical examinations are generally unremarkable, but if there are physical findings, further evaluation is necessary. Laboratory testing should include a complete blood count, erythrocyte sedimentation rate, chemistry panel, and thyroid function tests. If diarrhea is predominant, a stool evaluation for ova and parasites and a rectosigmoid mucosal biopsy should be included. Patients older than 40 and all patients with chronic diarrhea should undergo flexible sigmoidoscopy to rule out neoplasms or inflammatory bowel disease. Patient histories should include an evaluation of psychological factors, such as depression and anxiety, as potential contributors to the patient’s somatic complaints. In the absence of obvious signs of organic disease, a strategy of symptomatic treatment and careful follow-up should be initiated. Camilleri also recommends a symptom-based “therapeutic trial” of four weeks as part of the diagnostic work-up.