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  • How is bowel cancer diagnosed?


    Some people have tests for bowel cancer because they have symptoms. Others may not have any symptoms, but have a strong family history of bowel cancer or have received a positive result from a screening test.

    To diagnose bowel cancer, your GP will examine you. This will include a digital rectal examination. They will refer you to a specialist for further tests. The tests you have depend on your specific situation and may include:

    • general tests to check your overall health and body function
    • tests to find cancer
    • tests to see if the cancer has spread (metastasised)

    Some tests may be repeated during or after treatment to check how well the treatment is working. It may take up to a week to receive your test results. If you feel anxious while waiting for test results, it may help to talk to a friend or family member, or call Cancer Council 13 11 20 for support.

    Screening test for bowel cancer

    Screening is the process of looking for polyps or cancer in people who don’t have any symptoms. It is particularly important for bowel cancer, which often has no symptoms in its early stages.

    The faecal occult blood test (FOBT) looks for microscopic traces of blood in your stools, which may be a sign of polyps, cancer or another bowel condition. An FOBT does not diagnose cancer, but if it finds blood, your doctor will recommend you have a colonoscopy within 30 days.

    Everyone over 50 should have an FOBT every two years. Finding bowel cancer early can significantly improve the chance of surviving the disease. Through the National Bowel Cancer Screening Program, people aged 50 to 74 are automatically sent a free FOBT kit. A kit can also be purchased from some pharmacies.

    You complete the test at home. For more information, phone 1800 118 868 or see cancerscreening.gov.au.

    The FOBT is only for low-risk people without symptoms of bowel cancer. Anyone with symptoms of bowel cancer must talk to their doctor about having a colonoscopy or other tests. If you have a strong family history or a genetic condition linked to bowel cancer, the FOBT is not the right test for you. You and other family members may need screening colonoscopies.

    A screening colonoscopy is recommended for high-risk people at 50 years of age, or 10 years before the earliest age a family member was diagnosed with bowel cancer, whichever comes first. Your doctor will let you know how often the test should be repeated.

    General tests

    Physical examination

    Your doctor will examine your body, feeling your abdomen for any swelling.

    To check for problems in the lowest part of the bowel (anus and rectum), your doctor will insert a gloved finger into your anus and feel for any lumps or swelling. This is called a digital rectal examination (DRE).

    The DRE may be uncomfortable, but it shouldn’t be painful. Because the rectum is a muscle, it can help to try to relax during the examination. The pressure on the rectum might make you feel like you are going to have a bowel movement, but it is very unlikely that this will happen.

    Blood test

    You may have a blood test to assess your general health and to look for signs that suggest you are losing blood in your stools.

    The blood test may measure chemicals that are found or made in your liver, and check your red blood cell count. Low red blood cell levels (anaemia) are common in people with bowel cancer, but may also be caused by other conditions.

    Further tests to find cancer in the bowel

    The main test used to look for bowel cancer is a colonoscopy. Other tests that are sometimes used to diagnose bowel cancer include virtual colonoscopy and sigmoidoscopy.


    A colonoscopy examines the whole length of the large bowel. It is still possible, however, that small polyps may be missed, especially if they are behind one of the many folds in the bowel or the bowel is not completely empty.

    • Before a colonoscopy, you will have a bowel preparation to clean your bowel. On the day of the procedure, you will probably be given an anaesthetic so you don’t feel any discomfort or pain. This will also make you drowsy and may put you to sleep.
    • During the procedure, the 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. Carbon dioxide or air will be passed into the colon. Your doctor will look for abnormal tissue (such as polyps), and take a sample (biopsy) for further examination.

    A colonoscopy usually takes about 20–30 minutes. You will need to have someone take you home afterwards, as you may feel drowsy or weak. An occasional side effect of a colonoscopy is temporary flatulence and wind pain, 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 talk to you about the risks.

    Less commonly used tests

    Virtual colonoscopy. This uses a CT or MRI scanner to create images of the colon and rectum and display them on a screen. It is also 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. Virtual colonoscopy is covered by Medicare only in some circumstances when a colonoscopy isn’t feasible.

    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. Before the test, you may be given a light anaesthetic.

    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 carbon dioxide or air 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.

    Bowel preparation


    If any of these tests show you have bowel cancer, you will have additional tests to see if the cancer has spread to other parts of your body.

    Your blood may be tested for a protein called carcinoembryonic antigen (CEA). This protein is produced by some cancer cells. If blood test results show that you have a high CEA level, your doctor may organise more tests. This is because other factors, such as smoking or pregnancy, may also increase CEA levels. CEA levels may be retested after treatment to see if the cancer has come back.

    CT scan

    A CT (computerised tomography) scan uses x-rays to take many pictures of the inside of your body and then compiles them into one detailed, cross-sectional picture. A scan is usually done as an outpatient. Most people are able to go home as soon as the test is over.

    Before the scan, dye is injected into a vein to make the pictures clearer. This dye may make you feel hot all over and leave a strange taste in your mouth for a few minutes. You might also feel that you need to urinate, but this sensation won’t last long.

    During the scan, you will lie on a table that moves in and out of the CT scanner, which is large and round like a doughnut. Your chest, abdomen and pelvis will be scanned to check if the cancer has spread to these areas. The scan takes five to 10 minutes and is painless.

    The dye used in a CT scan usually contains iodine. If you have had an allergic reaction to iodine or dyes during a previous scan, let your medical team know beforehand. You should also tell them if you’re diabetic, have kidney disease or are pregnant.

    MRI scan

    An MRI (magnetic resonance imaging) scan uses a powerful magnet to build up cross-sectional pictures of the inside of your body. Only people with cancer in the rectum have an MRI; it is not commonly used for cancers higher in the bowel. An MRI may be used before surgery to stage rectal cancer or before radiotherapy.

    Before the scan, let your doctor know if you have a pacemaker or any other metallic object in your body. The magnet can interfere with some pacemakers, but some newer pacemakers are MRI-compatible. As with a CT scan, a dye might be injected into a vein before the scan to help make the pictures clearer.

    During the scan, you will lie on an examination table that slides into a large metal tube that is open at both ends. The noisy, narrow machine makes some people feel anxious or claustrophobic. If you think you may become distressed, mention it beforehand to your medical team. You may be given a medicine to help you relax and you will usually be offered headphones or earplugs.

    The MRI scan may take between 30 and 90 minutes, depending on the size of the area being scanned and how many images are taken.

    FDG-PET scan

    Medicare does not currently cover the cost of an FDG-PET (fluorodeoxyglucose-positron emission tomography) scan for bowel cancer, but this scan can find disease at sites that may not be picked up on a CT scan. If this test is recommended, check with your doctor what you will have to pay.

    Before an FDG-PET scan, you will be injected with a special modified sugar molecule (fluorodeoxyglucose or FDG). You will be asked to sit quietly for 30–90 minutes while the solution moves through your body.

    Your body is then scanned for areas with high levels of FDG. Cancer cells absorb more of the FDG, so they will be highlighted when your body is scanned.

    It will take several hours to prepare for and have the scan.

    This website page was last reviewed and updated November 2018

    Information reviewed by: A/Prof Craig Lynch, Colorectal Surgeon and Chair, Lower Gastrointestinal Cancer Service, Peter MacCallum Cancer Centre, VIC; Merran Findlay, Executive Research Lead–Cancer Nutrition, and Oncology Specialist Dietitian, Royal Prince Alfred Hospital, NSW; Jackie Johnston, Palliative Care and Stomal Therapy Clinical Nurse Consultant, St Vincent’s Private Hospital, NSW; A/Prof Susan Pendlebury, Radiation Oncologist, St Vincent’s Clinic, NSW; Jan Priaulx, 13 11 20 Consultant, Cancer Council NSW; A/Prof Eva Segelov, Professor of Oncology, Monash Health and Monash University, VIC; Heather Turner, Consumer; Lynne Wolowiec, Consumer.

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