To diagnose IBS, a health care provider will conduct a physical exam and take a complete medical history. The medical history will include questions about symptoms, family history of GI disorders, recent infections, medications, and stressful events related to the onset of symptoms. For IBS to be diagnosed, the symptoms must have started at least 6 months prior and must have occurred at least 3 days per month for the previous 3 months. Further testing is not usually needed, though the health care provider may do a blood test to screen for other problems. Additional diagnostic tests may be needed based on the results of the screening blood test and for people who also have signs such as:
- fever
- rectal bleeding
- weight loss
- anemia—too few red blood cells in the body, which prevents the body from getting enough oxygen
- family history of colon cancer, irritable bowel disease—long-lasting disorders that cause irritation and ulcers, or sores, in the GI tract—or celiac disease—an immune disease in which people cannot tolerate gluten, a protein found in wheat, rye, and barley, because it will damage the lining of their small intestine and prevent absorption of nutrients
Additional diagnostic tests may include a stool test, lower GI series, and flexible sigmoidoscopy or colonoscopy. Colonoscopy may also be recommended for people who are older than 50 to screen for colon cancer.
Stool tests. A stool test is the analysis of a sample of stool. The health care provider will give the person a container for catching and storing the stool. The sample is returned to the health care provider or a commercial facility and sent to a lab for analysis. The health care provider may also do a rectal exam, sometimes during the physical exam, to check for blood in the stool. Stool tests can show the presence of parasites or blood.
Lower GI series. A lower GI series is an x-ray exam that is used to look at the large intestine. The test is performed at a hospital or outpatient center by a radiologist—a doctor who specializes in medical imaging. The health care provider may give the person written bowel prep instructions to follow at home. The person may be asked to follow a clear liquid diet for 1 to 3 days before the procedure. A laxative or enema may be used before the test. A laxative is medication that loosens stool and increases bowel movements. An enema involves flushing water or laxative into the anus using a special squirt bottle.
For the test, the person will lie on a table while the radiologist inserts a flexible tube into the person’s anus. The large intestine is filled with barium, making signs of problems with the large intestine that may be causing the person’s symptoms show up more clearly on x rays.
For several days, traces of barium in the large intestine cause stools to be white or light colored. Enemas and repeated bowel movements may cause anal soreness. A health care provider will provide specific instructions about eating and drinking after the test.
Flexible sigmoidoscopy and colonoscopy. The tests are similar, but a colonoscopy is used to view the rectum and entire colon, while a flexible sigmoidoscopy is used to view just the rectum and lower colon. These tests are performed at a hospital or outpatient center by a gastroenterologist—a doctor who specializes in digestive diseases. For both tests, a health care provider will give written bowel prep instructions to follow at home. The person may be asked to follow a clear liquid diet for 1 to 3 days before either test. The night before the test, the person may need to take a laxative. One or more enemas may also be required the night before and about 2 hours before the test.
In most cases, light anesthesia, and possibly pain medication, helps people relax. For either test, the person will lie on a table while the gastroenterologist inserts a flexible tube into the anus. A small camera on the tube sends a video image of the intestinal lining to a computer screen. The test can show signs of problems in the lower GI tract.
The gastroenterologist may also perform a biopsy, a procedure that involves taking a piece of intestinal lining for examination with a microscope. You will not feel the biopsy. A pathologist—a doctor who specializes in diagnosing diseases—examines the tissue in a lab.
Cramping or bloating may occur during the first hour after the test. Driving is not permitted for 24 hours after a colonoscopy to allow the sedative time to wear off. Before the appointment, a person should make plans for a ride home. Full recovery is expected by the next day.