IBS is generally classified as a “functional” disorder, meaning that the body’s normal function is impaired but there are no obvious structural abnormalities or other physical signs that allow for a definitive diagnosis. Because of this, the diagnosis of IBS is largely based on a doctor’s examination and your personal and family medical history. However, because the symptoms of IBS overlap with many other conditions, your doctor may order any of several tests to help rule out these conditions as the cause of your symptoms.

The criteria for an IBS diagnosis include:

• Abdominal pain or discomfort for at least 12 weeks out of the previous 12 months (although the 12 weeks do not have to be consecutive).
• The abdominal pain or discomfort must have two of the following three features:
• It is relieved by having a bowel movement.
• When it starts, there is a change in how often you have a bowel movement.
• When it starts, there is a change in the form of the stool or the way it looks.
• Two of the following symptoms must also be present:
• A change in the frequency or appearance of your stool.
• Feelings of uncontrollable urgency to have a bowel movement.
• Straining, or a feeling that you can’t empty your bowels completely.
• Mucus in your stool.
• Bloating or abdominal distension.

Even if you meet these criteria, the doctor will probably want to confirm that you don’t have any other “red flag” symptoms that could be signs of something more serious (and may warrant further tests). These include: bleeding, fever, weight loss, recurrent vomiting, persistent severe pain, and new onset of symptoms after age 50. If you do not have any of these additional symptoms, the doctor will likely tentatively diagnose you with IBS and prescribe treatment. If the treatment fails, he or she will require more tests.

If you do have red flag symptoms or if the prescribed IBS treatment does not work, further tests may be needed to diagnose your condition. These include:

• Sigmoidoscopy: this test uses a flexible scope to examine the S-shaped part of the colon just above the rectum called the sigmoid colon.
• Colonoscopy: This test uses a small, flexible camera to examine the entire length of the colon.
• CT scan: Computed tomography (CT) scans of your the abdominal and pelvic regions can help show if anything is out of place in these areas.
• Lactose tolerance: Many people have problems digesting milk-sugar called lactose because they lack the right enzyme and this can cause symptoms similar to IBS. The doctor may ask you eliminate dairy products from your diet to see if this helps.
• Celiac disease tests: Celiac disease is sensitivity to a protein called gluten found in wheat, rye, and barley and it may also cause symptoms similar to IBS. Blood tests may help rule out that disorder.