Diagnosis of IBS

IBS is no longer regarded as a diagnosis of exclusion. Rather, IBS is diagnosed by a specific diagnostic criteria.

The diagnosis of IBS begins with a careful history. Abdominal pain must be present; the absence of abdominal pain precludes the diagnosis of IBS. Pain can be present anywhere throughout the abdomen, although it is more common in the lower abdomen. A history of constipation or diarrhea or both, should be found, along with the episodic association of abdominal pain. Unpredictable bowel pattern (different stool form types/week) reinforces the diagnosis of IBS in the diarrhea subtype (IBS-D). An increasing number of consecutive days without a bowel movement is associated with the diagnosis of constipation-predominant IBS (IBS-C). Excessive straining during defecation, urgency, feelings of incomplete evacuation, and mucus with bowel movements, although common in IBS, are not specific. Abdominal bloating is present in a majority of IBS patients; abdominal distention may be reported as well, although neither is required to make the diagnosis of IBS. It is important to distinguish Inflammatory Bowel Disease vs. Irritable Bowel Syndrome.

*Diagnostic Criteria for Irritable Bowel Syndrome

Recurrent abdominal pain, on average, at least 1 day per week in the last 3 months, associated with 2 or more of the following criteria:

1. Related to defecation
2. Associated with a change in frequency of stool
3. Associated with a change in form (appearance) of stool

Criteria fulfilled for the last 3 months with symptoms onset at least 6 months before diagnosis.

Note: There is a limitation to this diagnostic criteria as it may exclude patients who do not fully meet this criteria but who could be treated similarly. Meeting the criteria may not be necessary in the daily care of patients but still can serve as a useful guide to help with the identification of IBS.

People with IBS are more likely to be:

  • Female – approximately 70% of people with IBS are female (in western culture)
  • Younger than age 50 years

According to the Rome IV diagnostic criteria definition of IBS there are these different varieties based on symptoms:

  • IBS with predominant constipation (IBS-C)
  • IBS with predominant diarrhea (IBS-D)
  • IBS with mixed bowel habits, ie: constipation and diarrhea (IBS-M)
  • IBS unclassified (IBS-U)
IBS with predominant constipation (IBS-C): More than one- fourth (25%) of bowel movements with Bristol Stool Form Scale types 1 or 2 (separate hard lumps or susage-shapped but lumpy) and less than one-fourth (25%) of bowel movements with Bristol stool form scale types 6 or 7 (mushy fluffy-pieces with ragged edges or watery).

IBS with predominant diarrhea (IBS-D): more than one- fourth (25%) of bowel movements with Bristol Stool Form Scale types 6 or 7 (mushy fluffy-pieces with ragged edges or watery) and less than one-fourth (25%) of bowel movements with Bristol stool form scale types 1 or 2 (separate hard lumps or susage-shapped but lumpy).

IBS with mixed bowel habits (IBS-M): more than one- fourth (25%) of bowel movements with Bristol Stool Form Scale types 1 or 2 (separate hard lumps or susage-shapped but lumpy) and more than one-fourth (25%) of bowel movements with Bristol stool form scale types 6 or 7 (mushy fluffy-pieces with ragged edges or watery).

IBS unclassified (IBS-U): Patients who meet diagnostic criteria for IBS but whose bowel habits cannot be accurately categorized into 1 of the 3 groups above should be categorized as having IBS unclassified.

Note: IBS subtypes can only be confidently established when a patient is evaluated off medications used to treat bowel habit abnormalities and based on predominant bowel habit on days with abnormal bowel movements.

IBS patients frequently report that symptoms are induced or exacerbated by meals, although these symptoms are not specific enough to be included in IBS diagnostic criteria. The presence of alarm features (a positive family history of colorectal cancer, rectal bleeding in the absence of bleeding hemorrhoids or anal fissures, unintentional weight loss, or anemia) does not improve the performance of IBS diagnostic criteria.

A review of the diet, with special attention paid to consuming dairy products, wheat, caffeine, fruits, vegetables, juices, sweetened soft drinks, and chewing gum, because these can mimic or exacerbate IBS symptoms. A physical examination should be performed in every patient evaluated for IBS to look for obvious sources of abdominal pain and/or rectal bleeding, if present.

Limited blood work should be considered which includes a complete blood count (CBC). C-reactive protein or fecal calprotectin (a marker of intestinal inflammation) should be measured, as these tests are helpful in excluding IBD in patients with symptoms suggestive of nonconstipated IBS. Tests for celiac disease should be performed in patients with IBS-D and IBS-M whose symptoms do not improve with treatment. Stool analysis (bacteria, ova and parasites) may be useful if diarrhea is the main symptom, especially in developing countries where infectious diarrhea is prevalent.

A new blood test for diarrhea predominant IBS (IBS-D), IBS-Smart, tests for the presence of CdtB (cytolethal distending toxin B), a toxin caused from gastroenteritis and Vinculin, a human protein commonly found in the nerves and lining of the gut. It is known that acute gastroenteritis can lead to IBS, known as post-infectious IBS or PI-IBS. The presence of CdtB and vinculin appear to be highly predictive of a diagnosis of IBS-D.

A screening colonoscopy is indicated in patients 50 years and older in the absence of warning signs (45 years in African Americans). Colonoscopy is also indicated for the presence of alarm symptoms or signs, a family history of colorectal cancer and persistent diarrhea that has failed empiric therapy. Preparation for a colonoscopy is key to a successful procedure. Colonoscopy cleanliness is scored using the Boston Bowel Preparation Scale (BBPS).

Bile acid malabsorption may be the cause of persistent, watery diarrhea in some patients. Breath tests to rule out carbohydrate malabsorption may be useful in some patients with IBS symptoms and persistent diarrhea.

A psychological screening should be considered since there is research that suggests that psychological disturbance is associated with IBS, especially in patients who seek medical care, and psychosocial factors affect outcome. Regardless of care-seeking status, IBS is associated with more psychiatric distress and sleep disturbance.

Note: Bleeding, fever, weight loss and severe pain that does not go away are not symptoms of IBS and may suggest other problems. Talk to your doctor right away if you have these symptoms.


Irritable Bowel Syndrome in adults. BPJ 58 Feb. 2014
Gastroenterology 2016;150:1393–1407
IBSSmart.com - Gemelli Biotech
*IBS Diagnostic Criteria based on Rome IV
**Bristol Stool Form Scale Types by permission of Dr. KW Heaton and Norgine Pharma Ltd

Last updated on Jan 3, 2020