Colorectal Cancer Association of Canada - CCAC

Screening & Diagnostics > A guide to screening tests  

The contents of this document have been generously reviewed and approved by Dr. Tanya Chawla, Assistant Professor & Staff Radiologist, University Health Network/Mt. Sinai Hospital, Toronto, Ontario.

Colorectal Cancer Screening


The National Cancer Institute of Canada (NCIC) acknowledges colorectal cancer as the third most common cancer and the second most common cause of death from a cancer among men and women in Canada, second only to lung cancer. If, however, the cancer is detected early through screening, it may be highly treatable and need not be deadly. In a majority of cases, colorectal cancer is preventable and yet each year in Canada, thousands of people are diagnosed with advanced colorectal cancer. The majority of colorectal cancers begin as benign growths in the lining of the large bowel wall called adenomatous polyps. Over the years (at least ten years), these polyps grow in size and number, thereby increasing the risk that the cells in the polyps will become cancerous and invade the wall and move on to other organs. Approximately two thirds of these cancers are found in the large intestine and one third in the rectum. Early removal of these growths will prevent colorectal cancer from developing in the first place. Hence, identification and removal of polyps are key to preventing the development of colorectal cancer.

Clearly, being screened as part of a regular physical exam has the potential to save lives and patients who are experiencing symptoms related to colorectal cancer should not delay accessing a screening test nor should patients who are at higher risk of developing the cancer. Briefly, the risk factors for colorectal cancer are:

Age: the older you are, the more likely you are to develop colorectal cancer. Heredity: you are more likely to develop colorectal cancer if someone in your family, especially a first degree relative (child, sibling, parent) has been diagnosed with it Diet: a diet high in red meat and low in fruits and vegetables may increase you risk Weight: obesity and a lack of physical activity increase the risk Alcohol Consumption: alcohol, especially beer, may increase the risk. Lower rates of colorectal cancer have been found in those who drink no alcohol Smoking: smoking also increases the risk of developing colorectal cancer
Enough cannot be said about the importance of screening in preventing, detecting and curing colorectal cancer. It is simply the best line of defense when it comes to protecting oneself from this deadly disease. Screening is clearly the best way to stop colorectal cancer in its tracks or prevent it from developing in the first place which is why the Colorectal Cancer Association of Canada (CCAC) has developed the following content providing information on the various colorectal cancer screening tests as well as a summary of the recommended guidelines overseeing the administration of the screening tests. Simply click on one of the following headings to access its information.




    According to the American Cancer Society, screening or testing, is performed while the patient is feeling well – so as to find any abnormalities early, before signs and symptoms of disease occur. Screening for colorectal cancer allows for the early detection of cancer when it is highly curable, as well as the detection of growths (polyps) that might eventually become cancer. These polyps may be removed preventing the cancer from developing altogether. There are several tests used to screen for colorectal cancer and polyps. Appearing below is a summary of the various types.

    (i) Guaiac Fecal Occult Blood Test (gFOBT)

    One of the presentations of colon cancer is chronic blood loss in the stool. Sometimes, such blood loss is so minimal, it cannot be seen when the stool is inspected in the toilet. Hence, a stool sample may be collected which is returned to the doctor or lab to test for occult (hidden) blood. The guaiac fecal occult blood test uses the chemical guaiac to detect heme in stool. Heme is the iron-containing component of the blood protein hemoglobin. The other type of FOBT, called Fecal Immunochemical Test, is explained below.

    The idea behind the gFOBT is that blood vessels at the surface of larger colorectal polyps or cancers are often fragile and easily damaged by the passage of feces. The damaged vessels usually release a small amount of blood into the feces, but only rarely is there enough bleeding to be noticeable in the stool. This test, however, cannot determine whether the blood is from the colon or from other portions of the digestive tract (such as the stomach). Therefore, if the test is positive, a colonoscopy is required to determine if there is a cancer, polyp, or other cause of bleeding such as ulcers, hemorrhoids, diverticulosis (tiny pouches that form at weak spots in the colon wall) or inflammatory bowel disease (colitis).


    gFOBT is done with a take-home kit that can be used in the privacy of the patient’s home. People having this test will receive a kit with instructions explaining how to take a stool or feces sample at home (usually specimens from 3 consecutive bowel movements that are smeared onto small squares of paper). The kit should then be returned to the doctor's office or medical lab (usually within 2 weeks) for testing. Supplies will include a test kit, test cards, either a brush or wooden applicator, and a mailing envelope. The instructions below can be used as a guide, but the instructions on the kit might be a little different depending upon province and manufacturer. Provincial screening programs have specified laboratories to carry out the work on the sample and it is important to follow the specific instructions of the program when returning the sample, including the accompanying paper work.

    FOBT Instructions:

    Some foods or drugs can affect the test, so the doctor may suggest that the following foods be avoided before the test as well as cessation of these drugs:

    If this test finds blood, a colonoscopy will be required to look for the source. It is not sufficient to simply repeat the FOBT or follow up with other types of tests.

    Additional instructions on how to use the FOBT may be found at Ontario’s Colon Cancer Check website which can be accessed by clicking on the following link: [>]

    In summary, the pros and cons of accessing an FOBT are:

    (ii) Fecal Immunochemical Test (FIT or iFOBT)

    The other type of FOBT called immunochemical FOBT or FIT, uses antibodies to detect human hemoglobin protein in stool. Much like the gFOBT, the test detects the presence of blood in the stools but the main difference is that the fecal immunochemical test uses a more high-tech laboratory method to detect the presence of blood. For this reason, it may be a more accurate way to screen for blood in the stools than the fecal occult blood test. If blood is detected, the patient will require follow-up testing such as colonoscopy, to determine the reason for the presence of blood in the stools.

    FIT is performed in much the same way as the gFOBT, but some patients may find it easier to use because there are no drug or dietary restrictions (vitamins or foods do not affect the FIT) and sample collection may take less effort. This test reacts to part of the human hemoglobin protein, which is found on red blood cells. This test is also less likely to react to bleeding from parts of the upper digestive tract, such as the stomach. As with the FOBT, the FIT may not detect a tumor that is not bleeding. . And if the results are positive for hidden blood, a colonoscopy is required to investigate further.

    Supplies will include a test kit, test cards, long brushes, waste bags, and a mailing envelope. The kit will provide detailed instructions on how to collect the specimen. The instructions below can be used as a guide, but the kit instructions might be a little different. Provincial screening programs have specified laboratories to carry out the work on the sample and it is important to follow the specific instructions of the program when returning the sample including the accompanying paper work.

    (iii) Flexible Sigmoidoscopy
    In this test, the doctor performs an examination to view the inside of the lower colon and rectum (usually about the lower 2 feet) for polyps and cancers using a flexible sigmoidoscope (a thin, flexible, lighted instrument having the thickness of a finger with a small video camera located at its end). It is inserted through the rectum and into the lower part of the colon and images from the scope are viewed on a display monitor. If an adenoma is found, subsequent colonoscopy may be performed because sigmoidoscopy does not examine the entire colon and so is less reliable than colonoscopy for detecting polyps. Sedation is usually not used for sigmoidoscopy. A thorough cleansing of the lower colon is necessary for this test. The colon and rectum must be empty and clean so the doctor can view the lining of the sigmoid colon and rectum. A special diet may need to be adhered to for a day before the exam, such as drinking only clear liquids as well as the use of strong laxatives which promotes the evacuation of the bowel contents.

    A sigmoidoscopy usually takes approximately 10-20 minutes wherein the patient is placed on a table on their left side with their knees positioned near their chest. The sigmoidoscope is lubricated to make it easier to insert into the rectum and may, therefore, feel cold upon entry. Upon insertion, the scope may stretch the wall of the colon, which may cause bowel spasms or lower abdominal pain. Air will be placed into the sigmoid colon through the scope so the doctor can view the colon better. It is quite common and normal to feel pressure and slight cramping in the lower abdomen during the procedure. After the procedure, once the air leaves the colon, the discomfort is alleviated.


    If a small polyp is found during the test, the doctor may wish to remove it with a small instrument passed through the scope, which will then be sent to a lab to be looked at by a pathologist (biopsy). If precancerous growths (polyps) or cancerous growths are discovered during a sigmoidoscopy, they may be removed and then biopsied with an instruction to perform a follow-up colonoscopy at a later date to look for polyps or cancer in the rest of the colon.

    It is quite normal to see a small amount of blood in the first bowel movement after the test. Hence, do not be alarmed. On the other hand, should the colon have become punctured (a rare but possible complication), immediate reporting of the complication to the treating physician should be done.

    In summary, the pros and cons of accessing sigmoidoscopy are:


    (iv) Colonoscopy

    In this test, the rectum and entire length of the colon are examined using a lighted instrument called a colonoscope, essentially a longer version of a sigmoidoscope. The colonoscope is inserted through the rectum into the colon. It has a video camera on the end that is connected to a display monitor so the doctor can see and closely examine the inside of the colon. Special instruments can be passed through the colonoscope to remove (biopsy) any suspicious looking areas such as polyps, if need be. During colonoscopy, precancerous and cancerous growths throughout the colon can be found and removed or biopsied, including growths in the upper part of the colon, where they would be missed by sigmoidoscopy. A thorough cleansing of the colon is necessary before this test, and most patients receive some form of sedation to help keep them comfortable. Colonoscopy may be performed in a hospital outpatient department, or in a clinic. The colon and rectum must be empty and clean so the doctor can view their inner linings during the test. Laxatives (liquids, pills, or both) will be prescribed for the day before the test and possibly an enema on the morning of the test. Many patients consider the bowel preparation to be the most unpleasant part of the test, as it usually requires the patient to be in the bathroom quite a bit.

    Other instructions may be given as well. For example, the doctor may instruct that only clear liquids may be ingested (water, apple or cranberry juice, and any gelatin except red or purple) for a day or 2 before the exam. Plain tea or coffee with sugar is usually okay, but no milk or creamer is allowed. Clear broth, ginger ale, and most soft drinks or sports drinks are usually allowed unless they have red or purple food colorings, which could be mistaken for blood in the colon. Patients will also receive instructions for the morning of the test to abstain from eating or drinking anything after midnight the night before the test. Patients may need to arrange for someone to drive them home from the test because the sedative used during the test can affect their ability to drive.

    The test itself usually takes approximately 30 minutes, although it may take longer if a polyp is found and removed. Before the colonoscopy begins, a sedating medicine is administered (usually through the vein) to promote comfort and drowsiness during the procedure. Patients may likely be awake, but may not be aware of what is going on and may not remember the procedure afterward. Most people will be fully awake by the time they get home from the test.

    During the procedure, the patient is placed on their side with their knees flexed and a drape will cover them. The patient’s blood pressure, heart rate, and breathing rate will be monitored during and after the test. The colonoscope is lubricated so it can be easily inserted into the rectum. Once in the rectum, the colonoscope is passed all the way to the beginning of the colon, called the cecum. If not sedated, patients may feel an urge to have a bowel movement when the colonoscope is inserted or pushed further up the colon. To ease any discomfort, it may help to breathe deeply and slowly through the mouth. The colonoscope will deliver air into the colon so that it is easier for the doctor to view the lining of the colon and use the instruments to perform the test. Suction will be used to remove any blood or liquid stools. The doctor will look at the inner walls of the colon as they slowly withdraw the colonoscope. If a small polyp is found, the doctor may remove it. Some small polyps may eventually become cancerous and it is for this reason that they are removed. This is usually done by passing a wire loop through the colonoscope to cut the polyp from the wall of the colon with an electrical current. The polyp can then be sent to a lab to be checked under a microscope to see if it has any areas that have changed into cancer. If the doctor sees a larger polyp or tumor or anything else abnormal, a biopsy may be done. For this procedure, a small piece of tissue is taken out through the colonoscope. The tissue is looked at under a microscope to determine if it is a cancer, a benign (non-cancerous) growth, or a result of inflammation.


    The bowel preparation before the test can be unpleasant. The test itself may be uncomfortable, but the sedative usually prevents this, and most people feel normal once the effects of the sedative wear off. Some people may have gas pains or cramping for a while after the test. In some cases, people may experience low blood pressure or changes in heart rhythms due to the sedation during the test, although these are rarely serious. If a polyp is removed or a biopsy is performed during the colonoscopy, patients may notice some blood in their stool for a day or two after the test. Significant bleeding is slightly more likely with colonoscopy than with sigmoidoscopy, but it is still uncommon. In rare cases, continued bleeding might require treatment. Although colonoscopy is a safe procedure, on rare occasions the colonoscope can puncture the wall of the colon or rectum. This is called a perforation. It can be a serious complication and at times requires surgical repair. Possible complications should be discussed ahead of time with the treating physician.

    In summary, the risks and benefits associated with accessing colonoscopy are: