Investigation

Investigations should be limited to those that are likely to exclude serious alternative diagnoses and when negative support a positive diagnosis of irritable bowel syndrom (IBS).

The most important differential diagnoses are coeliac disease, colorectal carcinoma and colitis.

'Red Flag' features (age > 50 years or weight loss or occult blood in faeces or family history of colorectal carcinoma) should be considered indications for full colonoscopy (1).

Initial screening tests in primary care include (1,2):

• full blood count, ESR, CRP and liver function tests - useful tests to screen for inflammation and other pathology
• antibody testing for coeliac disease (endomysial antibodies or tissue transglutaminase)
• other possible investigations include (1):
  • thyroid function tests
  • faecal occult blood test (from three to six separate stool samples), and taken with adequate exclusion of meats from the diet - important to detect bleeding lesions in the gastrointestinal tract, and decide on the indication for colonoscopy

Subsequent investigation, if indicated, requires secondary care referral.

NICE suggest that the following tests are not necessary to confirm diagnosis in people who meet the IBS diagnostic criteria:

• ultrasound
• rigid/flexible sigmoidoscopy
• colonoscopy; barium enema
• thyroid function test - although this test is recommended by some (1)
• faecal ova and parasite test
• faecal occult blood - although this test is recommended by some (1)
• hydrogen breath test (for lactose intolerance and bacterial overgrowth)

Reference:

• (1) Best Pract Res Clin Gastroenterol. 2004;18(4):735-46
• (2) NICE (April 2017).Irritable bowel syndrome in adults - diagnosis and management of irritable bowel syndrome in primary care.