Irritable bowel syndrome (IBS) can be diagnosed based on symptoms. A doctor diagnoses IBS when a person has the typical symptoms of the disorder and, if needed, tests have ruled out other possible causes.
Most people won't need tests, but some people may because of their age and symptoms. The amount of testing you get depends on several things: your age, how your symptoms come on and how severe they are, and how you respond to your first treatment. For example, a 20-year-old might not need tests. But a 50-year-old with new symptoms might need tests because of the higher risk of colon cancer in people over 50.
Irritable Bowel Syndrome: Should I Have Tests for IBS Symptoms?