Most of the time, doctors can diagnose IBS from the symptoms. Your doctor will ask you about your symptoms and past health and will do a physical exam. A doctor diagnoses IBS when a person has the typical symptoms of the disorder and when tests, if needed, have ruled out other possible causes.

Most people won't need tests. But some people may need them 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.

Tests may include:

• A blood test for celiac disease.
• Complete blood count.
• Sedimentation rate, which checks for inflammation in the body.
• Stool analysis.

A thyroid function test or other tests, such as a colonoscopy, are sometimes done.