Since there are no blood tests or other examinations that are diagnostic for irritable bowel syndrome (IBS), this condition is generally diagnosed by exclusion. This means that gastrointestinal diseases that can also give rise to the symptoms exhibited in IBS are excluded by performing several tests and examinations that are diagnostic of those diseases.

The exclusion approach to reach the IBS diagnosis is expensive and subjects the patients to troublesome, racking, and often painful examinations. Therefore, several attempts have been made to achieve a positive diagnosis based on clinical symptoms similar to those used in psychiatric and rheumatic diseases. In 1988, these attempts culminated in the introduction of the Rome I criteria by an international panel of experts. These criteria were followed by refinements in 1999 (Rome II criteria) and in 2006 (Rome III criteria).

The criteria for the diagnosis of IBS as described by Rome III are intermittent abdominal pain/discomfort on at least 3 days per month in the last 3 months together with two of the following three criteria: improvement on defecation, change in stool frequency, or change in the stool form. To exclude other organic diseases with similar symptoms, patients should not express any additional potentially alarming symptoms such as rectal bleeding, anemia, loss of body weight, fever, or signs of inflammation. IBS patients are divided according to Rome III criteria into three subtypes on the basis of differences in the predominant bowel habits: diarrhea-predominant, constipation-predominant, or a mixture of both diarrhea and constipation. The main goal of this subdivision was to facilitate treatment of the symptoms. However, this subdivision is unstable and interchangeable, with more than 75 % of IBS patients switching from one subtype to another over time.