Diagnosis

There's no test to definitively diagnose IBS. Your doctor is likely to start with a complete medical history, physical exam and tests to rule out other conditions, such as celiac disease.

After other conditions have been ruled out, your doctor is likely to use one of these sets of diagnostic criteria for IBS:

• Rome criteria. These criteria include abdominal pain and discomfort lasting on average at least one day a week in the last three months, associated with at least two of these factors: Pain and discomfort are related to defecation, the frequency of defecation is altered, or stool consistency is altered.
• Type of IBS. For the purpose of treatment, IBS can be divided into three types, based on your symptoms: constipation-predominant, diarrhea-predominant or mixed.

Your doctor will also likely assess whether you have other signs or symptoms that might suggest another, more serious, condition. These signs and symptoms include:

• Onset of signs and symptoms after age 50
• Weight loss
• Rectal bleeding
• Fever
• Nausea or recurrent vomiting
• Abdominal pain, especially if it's not related to a bowel movement, or occurs at night
• Diarrhea that is persistent or awakens you from sleep
• Anemia related to low iron

If you have these signs or symptoms, or if an initial treatment for IBS doesn't work, you'll likely need additional tests.

Additional tests

Your doctor may recommend several tests, including stool studies to check for infection or problems with your intestine's ability to take in the nutrients from food (malabsorption). You may also have a number of other tests to rule out other causes for your symptoms.

Diagnostic procedures can include:

• Colonoscopy. Your doctor uses a small, flexible tube to examine the entire length of the colon.
• X-ray or CT scan. These tests produce images of your abdomen and pelvis that might allow your doctor to rule out other causes of your symptoms, especially if you have abdominal pain. Your doctor might fill your large intestine with a liquid (barium) to make any problems more visible on X-ray. This barium test is sometimes called a lower GI series.
• Upper endoscopy. A long, flexible tube is inserted down your throat and into the tube connecting your mouth and stomach (esophagus). A camera on the end of the tube allows the doctor to inspect your upper digestive tract and obtain a tissue sample (biopsy) from your small intestine and fluid to look for overgrowth of bacteria. Your doctor might recommend endoscopy if celiac disease is suspected.

Laboratory tests can include:

• Lactose intolerance tests. Lactase is an enzyme you need to digest the sugar found in dairy products. If you don't produce lactose, you may have problems similar to those caused by IBS, including abdominal pain, gas and diarrhea. Your doctor may order a breath test or ask you to remove milk and milk products from your diet for several weeks.
• Breath test for bacterial overgrowth. A breath test also can determine if you have bacterial overgrowth in your small intestine. Bacterial overgrowth is more common among people who have had bowel surgery or who have diabetes or some other disease that slows down digestion.
• Stool tests. Your stool might be examined for bacteria or parasites, or a digestive liquid produced in your liver (bile acid), if you have chronic diarrhea.