Irritable bowel syndrome (IBS) is a gastrointestinal disorder characterized by chronic and/or recurrent abdominal pain or discomfort and altered bowel habits. Although IBS is a common disorder well-known by many, diagnosis can be difficult.

Recommended diagnostic criteria are evolving and can vary between professional organizations. Many clinicians consider the Rome IV diagnostic criteria for IBS the gold standard for symptom-based diagnosis in adults. These criteria are also endorsed by the American College of Gastroenterology. Compared to the previous Rome III criteria, the Rome IV criteria use a higher symptom and frequency threshold for diagnosis; it now requires the presence of abdominal pain at least one day per week (on average), as opposed to abdominal discomfort at least three days per month. In addition, using the Rome IV diagnostic criteria, recurrent abdominal pain must be associated with two or more of the following for IBS diagnosis: changes in stool frequency, changes in stool form, or changes in relation to defecation. All Rome IV criteria must be present in the past three months with symptom onset at least six months prior to diagnosis.  

However, other professional societies, including the British Society of Gastroenterology, recommend basing IBS diagnosis in adults on criteria set forth by the National Institute for Health and Care Excellence (NICE), which allows consideration of an IBS diagnosis in patients with abdominal pain OR discomfort if the pain or discomfort is either relieved by defection or associated with altered bowel frequency or stool form. Associated symptoms required for IBS diagnosis also differ between NICE and Rome IV criteria, with NICE requiring at least two of the following: altered stool passage, passage of mucus, worsening of symptoms by eating, or abdominal bloating, distension, tension, or hardness. 

Despite their differences, both diagnostic criteria encourage what’s called a positive symptom-based diagnostic strategy, meaning diagnosing IBS is based on the presence of specific symptoms and not based on ruling out other conditions. There is a general agreement that a positive diagnostic strategy improves patient outcomes and avoids extensive, unnecessary testing. Blood tests, including complete blood count (CBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) or plasma viscosity test, and serological tests to screen for celiac disease, are recommended during initial IBS diagnosis. If these tests are normal, further testing is usually not warranted. Of course, in the case of patients with alarm features such as rectal bleeding, an abdominal or rectal mass, and/or unintended weight loss, testing to exclude cancer or serious disease is recommended. 

Both the NICE and Rome IV diagnostic criteria encourage characterizing adults with IBS by clinical subtype based on their predominant stool pattern: IBS with diarrhea (IBS-D), IBS with constipation (IBS-C), and IBS with mixed bowel habits (IBS-M). A categorization of IBS unclassified (IBS-U) is appropriate in adults without a significant abnormal stool pattern. It is important to note that IBS is a heterogeneous condition. Patients can have varying symptoms, and symptoms can change over time. Moreover, IBS subtypes can change and can be affected by treatment, highlighting the need for a strong patient-clinician relationship and routine follow-up.  

Although challenging, accurate diagnosis of IBS and identification of clinical subtype is critical to ensure that patients receive the most appropriate and effective treatment available. Read more about IBS diagnosis, including a full description of diagnostic criteria, in DynaMedex

To learn about management of IBS, see Irritable Bowel Syndrome (IBS) Awareness Month Part Two.

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