How is irritable bowel syndrome diagnosed?

Clinically ... Irritable Bowel Syndrome (IBS) is characterised by abdominal pain, bloating and a change in bowel habit, or frequency present for at least 6 months. The pain is often eased by passing a stool, symptoms may be worse with eating. Your GP will take a history, examine the abdomen and likely order some blood tests to exclude other causes of abdominal pain and altered bowel habit. See http://patient.info/ibshealth#content.
Irritable bowel synd. Irritable bowel syndrome (IBS) is relatively benign compared to inflammatory bowel disease (IBD). IBS is diagnosed clinically based upon Rome or Manning criteria (http://www.mayoclinic.org/diseases-conditions/irritable-bowel-syndrome/basics/tests-diagnosis/con-20024578). Depending upon age & other risk factors, some invasive testing eg colonoscopy may be performed but often diagnosis made clinical.