Diagnostic Tests to Examine the Lower GI Tract

Various tests are available to obtain an inner view of the lower digestive tract:

  • barium enema
  • sigmoidoscopy
  • colonoscopy
  • virtual colonoscopy
  • capsule endoscopy

All of these procedures can be performed on an outpatient basis at your doctor's office or a hospital. The colon must be emptied before all of these procedures so that your doctor can obtain a clear view of the inside of your lower digestive tract. Your doctor will give you specific instructions on how to do this, but the process typically involves consuming a liquid diet for up to three days before the procedure. Also, you will be told not to eat or drink anything after midnight the night before the test and to use laxatives the day before the procedure to cleanse the colon. On the morning of the procedure, an enema (a liquid solution passed into the anus) will be needed to cleanse the colon completely.

Barium Enema

Your doctor may order a barium enema, also called a lower gastrointestinal (GI) series, to detect structural problems at the far end of your bowel. If a colonoscopy cannot be performed, a barium enema may also be used to diagnose inflammatory bowel disease (Crohn's disease and ulcerative colitis) and colon cancer.

Barium sulfate—a type of contrast dye—is administered through the anus and into the colon to help produce x-ray images of the lower digestive tract. During the exam, you lie on your side with your knees bent toward your chest. A lubricated tube is gently inserted through the anus and into the rectum.

Barium is then passed through the tube into the rectum, allowing the colon to fill with contrast dye. No sedation or pain medication is needed, but you may feel some abdominal discomfort or an urge to pass stool during the test.

A radiologist will take continuous x-rays of your abdomen and view them on a screen. If any abnormalities are seen, the radiologist will take spot x-rays for later analysis. When the procedure is complete (it typically takes 30 to 45 minutes), you will be taken to the bathroom to expel some of the barium into the toilet.

If a buildup of barium prevents you from having a bowel movement in the days after the test, your doctor may recommend a laxative or an enema. Also, your stool may be whitish in color for up to three days after the procedure because of the barium. Serious complications, such as perforation of the colon, are rare.


During a sigmoidoscopy, a gastroenterologist examines the inside of your rectum and sigmoid colon (the last part of the large intestine) using a 2-foot, flexible viewing tube called a sigmoidoscope. You might have this test to screen for colorectal cancer (although a colonoscopy is the preferred method) or to investigate suspicious rectal bleeding, diarrhea, or pain.

Usually a sigmoidoscopy is done without a sedative, although you can request one if you feel anxiety or pain. While you lie on your left side, the doctor manually inspects the anus and rectum for any blockages and then gently inserts the sigmoidoscope into the anus, rectum, and lower colon to view the inner lining of these parts of the digestive tract. The physician may take biopsy samples and remove any polyps (abnormal growths).

The procedure takes up to 30 minutes, and, if no sedation was necessary, you usually can drive home afterward. If you do receive sedation, you won't be able to leave until it wears off, and you'll need someone to drive you home. Some people experience gas or a small amount of bleeding or abdominal cramping following a sigmoidoscopy. More serious complications, such as excessive bleeding and perforation of the colon or rectum, can occur but are rare.


A colonoscopy is similar to a sigmoidoscopy, but a longer flexible viewing tube called a colonoscope is used to examine the entire colon and (if necessary) the lower portion of the small intestine. It is typically performed to detect colorectal cancer and to determine the causes of rectal bleeding, chronic diarrhea, chronic constipation, or abdominal pain.

Pain medication and a mild sedative are usually given intravenously just before a colonoscopy. While you lie on the examining table, the doctor manually checks for blockages in the anus and rectum. He or she then gently inserts the tube through the anus and rectum and into the colon. As the colonoscope is slowly withdrawn, the physician inspects the lining of the intestines and may take biopsy samples of any abnormal tissue and remove any polyps.

You may experience abdominal cramping, bloating, and a need to pass stool or gas during the procedure. The whole process can take up to an hour. The sedation will wear off about an hour after the procedure. You'll need to have someone drive you home because of the lingering effects of the sedative. It is common to have gas after the procedure and to have minor rectal bleeding if a biopsy was performed. Serious complications are rare (occurring in fewer than 0.1% of all colonoscopies) and can include excessive bleeding, labored breathing (from the sedative), and perforation of the colon or rectum.

Virtual Colonoscopy

A virtual colonoscopy requires the same preparation to empty the bowels as a standard colonoscopy, but instead of inserting a viewing tube into the colon and rectum, a CT scan of the abdomen is done to obtain two- and three-dimensional images of the intestines. Because the test will cause only minor discomfort, sedation is not needed.

You might be wondering why your doctor wants to perform a standard colonoscopy when a virtual colonoscopy sounds like an easier test to endure. Even though a three-dimensional CT virtual colonoscopy is considered to be as accurate as a standard colonoscopy for detecting abnormal growths in the colon and rectum, the test has a number of drawbacks.

First, biopsy samples cannot be taken with a virtual colonoscopy. This means that if any abnormalities are detected, you will need to undergo a standard colonoscopy for confirmation of the diagnosis. Second, polyps cannot be removed during virtual colonoscopy as they can during a standard colonoscopy. Third, virtual colonoscopy may not be able to distinguish stool from polyps or cancers.

Because of these drawbacks, your doctor won't perform a virtual colonoscopy unless you have a medical condition that makes a standard colonoscopy risky or physically difficult.

Capsule Endoscopy

When symptoms, such as gastrointestinal bleeding or chronic abdominal pain, cannot be explained using standard diagnostic procedures, capsule endoscopy may be a useful technique. It allows for a full view of the small intestine, particularly the areas that are usually unreachable with an upper endoscopy or colonoscopy. However, capsule endoscopy cannot be used to view the esophagus or the stomach, and biopsy samples cannot be taken.

Similar to the other imaging techniques described above, you will need to follow a liquid diet and take a laxative to empty your bowels before the procedure. On the day of the procedure, you will swallow a camera-containing capsule with a full glass of water. The capsule is then propelled through the digestive tract by peristalsis.

You can go about your normal daily activities, although you must wait two hours before drinking clear liquids and four hours before eating a light meal.

The capsule is somewhat larger than a vitamin pill and contains a video camera, light, and radio transmitter. It takes pictures (two per second) of the digestive tract and transmits these images to a Walkman-like device that is worn at your waist. After about eight hours, you return the recording device to your doctor, who downloads the information from the device to a computer.

The capsule is eliminated in your stool and does not need to be returned. Your doctor, however, may ask you to watch for the capsule in your stool; it gets stuck in the intestines of about 0.5% of people.

Publication Review By: H. Franklin Herlong, M.D.

Published: 24 Mar 2011

Last Modified: 11 Sep 2015