Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by abdominal pain and altered bowel habits, either constipation or diarrhea, in the absence of any identifiable abnormality in the gastrointestinal tract. It accounts for approximately 25%-50% of outpatient gastroenterology referrals, and it is the second highest cause of work absenteeism in the United States. IBS has a female predominance.

IBS is a diagnosis of exclusion, as it has no histologic phenotype and no sensitive or specific laboratory tests. However, recent American College of Gastroenterology guidelines recommend a diagnostic testing strategy to expedite the time of diagnosis, decreasing delays in treatment (see References section).

Rome III diagnostic criteria require recurrent abdominal pain at least 1 day per week over 3 months, associated with 2 of the following:

• Pain related to defecation
• Change in frequency of stool (constipation or diarrhea)
• Change in appearance of stool

Manning diagnostic criteria require at least 2 of the following:

• Onset of pain linked to more frequent bowel movements
• Looser stools associated with the onset of pain
• Pain relieved by passage of stool
• Noticeable abdominal bloating
• Sensation of incomplete evacuation more than 25% of the time
• Diarrhea with mucus more than 25% of the time

Four subtypes of IBS are recognized:

• IBS with constipation – Hard / lumpy stools > 25% and loose / watery stools < 25%
• IBS with diarrhea – Loose / watery stools > 25% and hard / lumpy stools < 5%
• Mixed IBS – Hard / lumpy stools > 25% and loose / watery stools > 25%
• Un-subtyped IBS – No consistent pattern of stool abnormality

Onset of IBS can occur at any age and is not curable. It does not progress, and patients can often find symptom relief through a variety of treatment options.