Irritable bowel syndrome (IBS) is a functional bowel condition which affects 20% of people. It affects women twice as commonly as men. The most common age of onset is between 20 and 30 years of age.

The most common symptoms include cramping abdominal pain, diarrhea or constipation (which may alternate), bloating, and nausea.

Symptoms tend to fluctuate in intensity, with remissions lasting from days to months. Some people are more severely symptomatic than others. Certain foods and stress appear to precipitate the symptoms.

Diagnosing IBS

In many cases, IBS can be diagnosed based purely on the symptoms. In a few cases, blood tests may be ordered to rule out other conditions. For instance, celiac disease or bowel infection may present with many of the same symptoms as IBS. The condition called inflammatory bowel disease (IBD) may also appear to be similar to IBS, and a stool test for the substance called calprotectin will distinguish the two.

Diagnosis of IBS depends upon the presence of criteria and the presence of red flag (or alarm) symptoms, such as weight loss, bleeding from the digestive tract, anemia, fever and symptoms occurring during the night.

Widely accepted criteria for diagnosis of IBS are the Rome II criteria that highlight the presence of abdominal pain for 12 weeks (not necessarily consecutive) in the foregoing 12 months, and at least two of the following three features: relief after defecation, changes in defecation frequency, and/or changes in the stool form or appearance.

If all criteria are met, and there are no alarm symptoms, the patient’s age should be considered:

• Those who are 50 years or less should be evaluated for their main symptoms
• Older patients should receive a full assessment before diagnosis

If all criteria are not met, and there are no alarm symptoms, the patient may be treated symptomatically and re-assessed in a few months. If alarm symptoms are present, any patient should be referred for a full evaluation.

Differential diagnosis

Common conditions which may be mistaken for IBS include:

• Inflammatory bowel disease as in Crohn's disease (ulcerative colitis)
• Symptoms which suggest obstruction of the intestine, called intestinal pseudo-obstruction, as in diabetes or scleroderma
• Abuse of medications such as laxatives or bowel binders
• Lactose intolerance
• Psychiatric disorders (such as depression, anxiety or somatization disorder)
• Infections of the digestive tract
• Malabsorption syndromes (such as celiac disease or pancreatic insufficiency)
• Endocrine disorders (such as hypothyroidism, hyperthyroidism, diabetes or Addison’s disease)
• Certain rare endocrine tumors (such as gastrinomas or carcinoid tumors)
• Carcinomas of the intestine

To differentiate between these, attention should be paid to the criteria, the presence of serious symptoms, and diagnostic testing as required.

Classifying the primary symptom

What is the predominant symptom – diarrhea, constipation, or pain? Based on the presence of other risk factors, constipation may suggest problems of the anorectal tract or the muscle of the large bowel. Conversely, diarrhea may suggest IBD, tumors or vascular malformations, ulceration, or metabolic conditions, as well as malabsorption. When pain is the most dominant symptom, obstruction should be thought of.

Laboratory testing and endoscopy

Blood tests are directed at assessing the general condition of the patient, as well as ruling out other disorders. Thus alterations in the complete blood count and erythrocyte sedimentation rate (ESR) will point towards several conditions such as inflammatory bowel disease, cancer of the digestive tract, and metabolic conditions which could result in gut symptoms. Electrolyte assessment will also show if diarrhea is severe enough to cause metabolic derangements.

Thyroid tests and stool tests for occult blood and several gut infestations will also help to rule out specific conditions which could cause similar symptoms, such as constipation in hypothyroidism, and abdominal cramps with parasitic infestations. If diarrhea is a prominent symptom, lactose intolerance will be tested for. Metabolic causes must also be looked for.

Endoscopy is of chiefly negative value in IBS, by ruling out organic disease in the upper digestive tract. Flexible sigmoidoscopy is adequate in most such cases, whereas symptoms which could signal more serious conditions, or a patient age above 50 years, should point to the need for a complete colonoscopy instead.