Health professionals usually diagnose IBS after discussing someone's symptoms and examining them. If the diagnosis is uncertain further tests may be advised.

One of these tests is called a sigmoidoscopy which is an examination of the lower part of the colon. Alternatively a biopsy may be done by removing a small piece of tissue from the bowel lining for examination. A biopsy can help to exclude inflammatory bowel disease, and certain symptoms mean that more tests are needed to rule out other conditions.

These symptoms can include weight loss, passing blood or mucus or general ill.

Another test that may be recommended is a colonoscopy, which allows the doctor to see the lining of the large bowel directly. Other tests include stool testing and X-rays of the bowel, taken after a barium enema (when the colon is filled with a liquid that shows up on X-rays).