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Alternate Names

  • sigmoidoscopy


A colonoscopy is a test in which a flexible fiberoptic instrument is inserted through the anus into the colon, also called the large intestine. This instrument, called a colonoscope, is a long, thin tube with a camera and light on the end. With it, a doctor can view the inner surface of the colon. The doctor can also sample or remove abnormal growths through the colonoscope.

Who is a candidate for the test?

A colonoscopy may be recommended for a person with:
  • abdominal pain
  • a change in bowel habits
  • colorectal polyps, which are small growths on the intestinal wall that may lead to cancer
  • a history of colorectal cancer
  • mucus, pus, or blood in the stool
  • prolonged or unexplained diarrhea
  • ulcerative colitis
A colonoscopy may also be recommended for someone at high risk for colorectal cancer. This may include a person who has a strong family history of colorectal cancer or polyps.

How is the test performed?

The person may be given medicine to make him or her drowsy or more comfortable during the procedure. The person lies on one side with knees flexed toward the abdomen. The doctor inserts the colonoscope through the anus and up into the large intestine. The instrument is pushed through the colon until it comes to the place where the colon meets the small intestine.
At that point, air is passed through the colonoscope to gently inflate the colon. This gives the doctor a clear view of the inner lining of the colon. The doctor then withdraws the instrument slowly, while examining all regions of the colon along the way. Places of interest on the interior of the colon are sometimes photographed.
If the doctor sees tissue that looks abnormal, a biopsy, which is a small tissue sample, may be taken. Small colorectal polyps can also be removed through the colonoscope.

What is involved in preparation for the test?

The doctor can provide instructions. Some type of bowel preparation is usually needed. The person may be asked to:
  • fast overnight before the test
  • have enemas to remove stool from the lower intestine
  • stay on a liquid diet for 1 to 3 days before the test
  • swallow a liquid or take tablets that cleanses the bowel

What do the test results mean?

Abnormalities in the lining of the colon may include:
  • colorectal cancer
  • colorectal polyps
  • diverticulosis, which are pockets in the intestinal walls
  • gastrointestinal bleeding
  • inflammatory bowel disease
  • abnormal blood vessels

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