Tests to find cancer in the bowel

The main test used to look for bowel cancer is a colonoscopy. Less commonly, some people have a CT colonography or flexible sigmoidoscopy.

Learn more about:

• Colonoscopy and biopsy
• Less commonly used tests

Colonoscopy and biopsy

A colonoscopy lets your doctor look at the lining of the entire large bowel. Before the procedure, you will have a bowel preparation to clean your bowel. It is very important to follow the instructions – the cleaner the bowel is, the more likely it is that the doctor will see any polyps or other areas of concern.

Most colonoscopies are done as day surgery at a hospital. On the day of the procedure, you will usually be given a sedative or light anaesthetic so you don’t feel any discomfort or pain. This will also make you drowsy and may put you to sleep. A colonoscopy usually takes about 20–30 minutes.

During the procedure, the doctor will put a colonoscope (a flexible tube with a camera on the end) through your anus and up into the rectum and colon. Carbon dioxide or air will be passed through the colonoscope to inflate the colon and make it easier for the doctor to see the bowel.

If the doctor sees any abnormal or unusual-looking areas, they will remove a sample of the tissue. This is known as a biopsy. They will also remove any polyps (polypectomy). A pathologist will examine the tissue or polyps under a microscope to check for signs of cancer and may look for specific gene changes (see Molecular testing).

Side effects

You will need to have someone take you home afterwards, as you may feel drowsy or weak, and you shouldn’t drive for at least 24 hours after the procedure. The gas used to inflate the bowel during the test can sometimes cause bloating and wind pain. Rare complications include damage to the bowel (perforation), damage to the spleen or bleeding. Your doctor will talk to you about the risks.

Less commonly used tests

CT colonography

Also called virtual colonoscopy, this uses a CT scanner to create images of the colon and rectum. Bowel preparation is usually needed before the test. A CT colonography is done by a radiologist, a specialist who analyses x-rays and scans.

You may have a CT colonography if a colonoscopy didn’t show all of the colon or when a colonoscopy is not safe. However, a CT colonography is not often used because it exposes you to radiation and is not as accurate as a colonoscopy. It can see only bigger polyps, not small ones.

If any abnormality is detected, you will need to have a colonoscopy so the doctor can take tissue samples. A CT colonography is covered by Medicare only in limited circumstances.

Flexible sigmoidoscopy

This test is similar to a colonoscopy but only lets the doctor see the rectum and lower part of the colon (sigmoid and descending colon). Before a flexible sigmoidoscopy, you will need to have a light bowel clean-out, usually with an enema. Just before the procedure, you may be given a light anaesthetic.

You will then lie on your left side while a colonoscope (or, sometimes, a shorter but similar tube called a sigmoidoscope) is put into your anus and guided up through the bowel. The colonoscope or sigmoidoscope blows carbon dioxide or air into the bowel to inflate it slightly so the doctor can see the bowel wall more clearly.

A light and camera at the end of the colonoscope or sigmoidoscope show up any unusual areas, and your doctor can take tissue samples (biopsies).

→ READ MORE: Further tests for bowel cancer

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