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Diagnostic Methods


People who are concerned about bowel cancer will usually consult their general practitioner (GP). If their GP thinks a person's concerns warrant specialist investigation, they will usually arrange referral to a gastroenterologist or colorectal surgeon for a colonoscopy.

Colonoscopy should not be used as a screening test for people with no symptoms or at average risk of bowel cancer. Australians aged 50 to 74 at average risk of bowel cancer (e.g. without symptoms, family history or precursor conditions) are urged to participate in Australia's National Bowel Cancer Screening Program. Program information is available here.

What is a colonoscopy?

A colonoscopy is the main test used to investigate or diagnose bowel cancer. Other tests used to diagnose bowel cancer include virtual colonoscopy and sigmoidoscopy.

A colonoscope is a tiny camera on a long flexible tube that is used to examine the entire length of the large bowel. 

Colonoscopy preparation

Before you have a colonoscopy, you will have a bowel preparation to clean your bowel.

The process varies for different people and between hospitals, so be sure to ask if there are any specific instructions for you.

It's important to follow the instructions you are given so you don't have to repeat the test. If you have any concerns about the bowel preparation process ensure you talk to you doctor about your concerns. 

Some of the things you may be asked to do include:

Changing your diet 

For a few days before the colonoscopy, you may be advised to avoid high-fibre foods, such as fruit, vegetables, brown rice, wholegrain pasta, bran, cereals, nuts and seeds. Instead, choose low-fibre options, such as white rice, white bread, meat, chicken, fish, yoghurt, cheese, potato and pumpkin. 

Taking prescribed laxatives

You may be prescribed a strong laxative to take 12 to 18 hours before the test. This is taken by mouth in liquid or tablet form, and will cause you to have watery diarrhoea that will help you empty your bowel.

Drinking clear fluids 

Your doctor may advise you to also drink only fluids, such as water, broth, black tea and coffee, and clear fruit juice (without pulp) for 12 to 24 hours before the test. This will help to prevent you from getting dehydrated. 

Ask if you need an enema 

A common way to clear the bowel is by using an enema. This involves inserting liquid directly into the rectum. The enema solution washes out the lower part of the bowel, and is passed into the toilet along with any faeces. 

How is a colonoscopy performed?

During the colonoscopy, your doctor will insert a flexible tube with a camera on the end (called a colonoscope) into your anus and up into your rectum and colon. Air or carbon dioxide will be passed into the colon. The doctor will look for abnormal tissue (such as polyps), and take a sample (biopsy) for further examination. The process usually takes about 20-30 minutes.

On the day of the procedure, you will likely be given an anaesthetic so you don't feel any pain or discomfort. As you may feel drowsy or weak prior to and after the colonoscopy it is best to arrange someone to drop you off at the hospital before the procedure and take you home afterwards. Some hospitals will not let you drive or leave unless someone is there to pick you up.

After a colonoscopy

Most people who have a colonoscopy do not have bowel cancer. However, if referred, it is important that you have the procedure to be sure. If a colonoscopy reveals anything of concern (such as polyps, which may or may not be potentially cancerous), a sample will be taken to be tested by a pathologist. The person may also need to have imaging (such as abdominal scans) before deciding the best type of treatment for the bowel cancer.

Sometimes the first sign of bowel cancer is sudden blockage of the bowel. When this happens, bowel cancer is diagnosed by x-ray or computed tomography (CT) scan and usually requires an emergency operation.

After bowel cancer is diagnosed, doctors work out what stage it is at (how far it has spread). This may be done by checking findings of the scans. Tissue taken at the time of colonoscopy may be tested for genetic changes in the cancer cells, which can influence choice of treatment. The health professionals treating the individual will work closely together to get an accurate understanding of the cancer.

Less commonly used diagnostic tests 

Virtual colonoscopy

This procedure uses a MRI or CT scanner to create images of the colon and rectum and display them on a screen. It is sometimes known as CT colonography.

A virtual colonoscopy is not often used because it is not as accurate as a colonoscopy and involves exposure to radiation. Your doctor may not be able to see small abnormalities and cannot take tissue samples.

A virtual colonoscopy is only performed in rare cases where a colonoscopy isn't possible. 

Flexible sigmoidoscopy

This test allows the doctor to see the rectum and lower part of the colon (sigmoid colon) only. To have a flexible sigmoidoscopy, you will need to have an empty bowel. You may be given a light anaesthetic before the test.

You will lie on your side while a thin, flexible tube called a sigmoidoscope is inserted gently into your anus and guided up through the bowel. The sigmoidoscope blows air or carbon dioxide into the bowel to inflate it slightly and allow the doctor to see the bowel wall more clearly. A light and camera, at the end of the sigmoidoscope show up any unusual areas, and tissue samples (biopsy) can be taken.

Your doctor may require additional tests to be performed to confirm any unusual areas in the bowel.

Are there any side effects?

An occasional side effect of a colonoscopy is temporary wind pain and flatulence, especially if air rather than carbon dioxide is passed into the bowel during the test. More serious but rare complications include damage to the bowel or bleeding. Your doctor will discuss the risks with you.

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