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A colonoscopy is an internal examination of the colon (large intestine), using an instrument called a colonoscope.

How the Test is Performed

The colonoscope is a small camera attached to a flexible tube. Unlike sigmoidoscopy, which examines only the lower third of the colon, colonoscopy examines the entire length of the colon.

You will lie on your left side with your knees drawn up toward the chest. After you've received a sedative and pain reliever, the colonoscope is inserted through the anus and gently advanced to the lowest part of the small bowel.

Air will be inserted through the scope to provide a better view. Suction may be used to remove secretions.

Since better views are obtained as the colonoscope is pulled back out, a more careful examination is done during withdrawal of the scope. Tissue samples may be taken with tiny biopsy forceps inserted through the scope. Polyps may be removed with electrocautery snares, and photographs may be taken.

Specialized procedures, such as laser therapy, may also be done.

How to Prepare for the Test

Thorough cleansing of the bowel is mandatory. Instructions for doing this will be given by the provider. This will include using enemas, not eating solid foods 2 or 3 days before the test, and taking laxatives. You may be told to stop taking aspirin or other blood-thinning medications for several days before the test.

To avoid dehydration, drink plenty of clear liquids such as juices and broths. Unless otherwise instructed, continue taking any regularly prescribed medication. Stop taking iron preparations a few weeks before the test, unless otherwise instructed by the health care provider. Iron residues produce a dark black stool, which makes the view inside the bowel less clear.

People with heart valve disease may receive antibiotics before and after the test to prevent infection. Outpatients must plan to have someone take them home after the test, as they will be woozy and unable to drive.

Infants and children:

The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

How the Test Will Feel

The sedative and pain medication will provide relaxation and produce a drowsy feeling. A rectal examination usually precedes the test to dilate the rectum and make sure there are no major obstructions. You may have the urge to defecate when the rectal exam is performed or as the colonoscope is inserted.

You may feel pressure as the scope moves inside. Brief cramping and gas pains may be felt as air is inserted or as the scope advances. The passing of gas is necessary and should be expected.

Discomfort may be lessened by taking slow, deep breaths. This will also help relax the abdominal muscles. Mild abdominal cramping and considerable passing of gas may occur after the exam. Sedation should wear off in a few hours. Because of the sedation, you may not feel any discomfort and may have no memory of the test.

Why the Test is Performed

  • To evaluate unexplained anemia
  • To evaluate unexplained blood in the stool, abdominal pain, persistent diarrhea, or abnormalities (such as polyps) found on contrast x-rays (barium enema)
  • To determine the type and extent of inflammatory bowel disease (ulcerative colitis and Crohn's disease)
  • To follow a previous finding of polyps, colon cancer, or a family history of colon cancer
  • To obtain tissue specimen for biopsy
  • To screen for colorectal cancer; Americans at average risk should begin screening at age 50.

Normal Results

Normal findings are healthy intestinal tissues.

What Abnormal Results Mean

  • Diverticulosis (abnormal pouches on the lining of the intestines, most often seen in older people)
  • Inflammatory bowel disease
  • Lower gastrointestinal (GI) bleeding
  • Polyps (which can be removed through the colonoscope during the exam)
  • Tumor

Additional conditions under which the test may be performed:


  • Bowel perforation (a hole or tear in the wall of the colon), requiring a repair operation (fewer than 1 out of 1,000 tests)
  • Heavy or persistent bleeding from biopsy or polyp-removal sites (1 out of 1,000 tests)
  • Adverse reaction to sedative medication, causing breathing problems or low blood pressure (4 out of 10,000 tests)
  • Infection requiring antibiotic therapy (very rare)
  • Nausea, vomiting, bloating, or rectal irritation caused by medicines, taken by mouth, that cleanse the bowel


You must sign an informed consent form. Several hours rest is recommended after the test. To replace fluids lost because of laxatives and fasting, drink plenty of liquids after the test.


Pasricha PJ. Gastrointestinal endoscopy. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2008:chap 136.

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Review Date: 3/8/2008
Reviewed By: Christian Stone, MD, Division of Gastroenterology, Washington University in St. Louis School of Medicine, St. Louis, MO. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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