How To Prepare For An At Home Colon Cancer Screening Test

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    If you are over the age of 50 or have a family history of colorectal cancer or polyps, you likely will be asked by your physician to get screened for colorectal cancer (CRC). A colonoscopy is considered the most accurate method for detecting colon cancer yet many people choose to avoid this invasive procedure and skip the screening altogether. However, screening has been shown by the CDC (Centers for Disease Control and Prevention) to be the most effective method to reduce your risk for colorectal cancer and is responsible for a 30% reduction in CRC mortality rates in the past 50 years. In choosing to skip the screening people may unknowingly be increasing their risk for developing and dying from colon cancer.

    Fortunately, if you are reluctant to get a colonoscopy you have options. In addition to colonoscopies, there are several screening tests available to you that find polyps or  detect colorectal cancer. These include:

    • Stool DNA tests 
    • Fecal occult blood stool test (FOBT)
    • Fecal immunochemical test (FIT)
    • Flexible sigmoidoscopy
    • CT (computerized tomography) colonography
    • DCBE (double contrast barium enema)

    Among these choices are at-home cancer screening tests which require no bowel preparation nor invasive procedure. Your healthcare provider may recommend that you take an at-home CRC screening test if you have no personal or family history of polyps and colon cancer. Currently there are three at-home options available to you by prescription: 

    • FIT test – The fecal immunochemical test (FIT) detects blood in the stool by using antibodies. The test is simple: you collect a small amount of fecal matter from a bowel movement, place it in a sterile container, and mail it to a lab for analysis
    • Multitarget stool DNA test (Cologuard™) – Like FIT, you collect a small amount of your stool and place it in sterile container to be shipped to a lab to analyse for cancer. This test detects not only blood in your stool, but also cellular changes to your DNA that indicate abnormal tissue growth. 
    • Guaiac-Based Fecal Occult Blood test (gFOBT) – This test uses the chemical guaiac to detect blood in the stool. It requires that you take three separate stool samples that are smeared on test cards to be mailed in for testing. 

    Your doctor may prescribe one of the three options mentioned above – all of which require you to take a sample of your stool from home without the uncomfortable bowel preparation. While these tests do not ask you to clear your bowels nor eat only clear liquids before you gather your sample, you may need to restrict certain foods or medications. 

    These restrictions may increase the accuracy of the test as some foods can cause false-positive and false-negative results.This article will help answer any questions you have about preparing for your at-home CRC screening test, including an explanation of why some foods interfere with screening results. It is important that you ask your healthcare provider if you need to restrict your diet and how to prepare for your test personally. Each healthcare office has its own set of guidelines for your screening test. . 

    Are there any restrictions to what foods or medications I can take before the test?

    You may need to avoid certain foods and pain-relieving medications depending on the test your doctor ordered. The FIT and Cologuard™  tests have no dietary restrictions and do not require any preparation. The fecal occult blood test, or FOBT, is a chemically sensitive test and does require you to restrict your diet a couple days before the test. This is because certain foods and medications can trigger false-positive or false-negative results.

    False-positive – This term is used by the medical community when a person tests positive for a condition or disease and does not have it. If you receive a false-positive from your home screening test, this means you test positive for cancer or blood in your stool when you in reality have neither.  

    False-negative – Conversely, this result means you actually have the disease or condition and the test does not detect it. A false-negative stool test means the lab failed to detect blood or abnormal cells in your stool. 

    Many people can receive a false-positive result because they have blood in their stools from hemorrhoids or a bleeding ulcer in their upper intestinal tract. Other people may receive a false-positive result because they have consumed an iron supplement or iron-rich foods such as red meat that contain compounds that look like hemoglobin, the oxygen-carrying protein found in our blood. 

    In order to avoid these inaccurate results, your doctor may ask you to avoid certain foods and supplements before you take your test.

    The fecal occult blood test (FOBT) carries the most restrictions due to the chemical reaction the test uses to detect blood in the stool. Many foods can interfere with this reaction and are listed below.  

    What foods or medications may cause my FOBT results to come back false-positive?

    Drugs and substances

    • Aspirin
    • Nonsteroidal anti inflammatory drugs (NSAIDs)
    • Boric acid
    • Bromides
    • Colchicine
    • Iodine

    Foods and supplements

    • Red meat
    • Beets
    • Carrots
    • Cauliflower
    • Mushrooms
    • Broccoli
    • Radishes
    • Turnips
    • Horseradish
    • Cantaloupe
    • Iron supplement

    What foods or medications cause my FOBT results to come back false-negative?

    • Vitamin C – This powerful antioxidant interferes with the chemical reaction that detects the presence of blood through a color change.
    • Iron supplement that contains at least 250mg of vitamin C

    When do these restrictions for FOBT start? What can happen if I do not follow them?

    Your doctor will tell you which foods and medications to avoid in the few days before you take your test. Remember that not follow these instructions may result in you getting a false-positive or false-negative result which requires further testing. 

    The typical timeline of restrictions and dietary guidelines is as follows:

    48-72 hours before sample collection

    Foods to eat:

    • High fiber foods, such as whole wheat bread and bran cereal
    • Cooked fruits and vegetables
    • Well-cooked poultry and fish

    Foods and supplements to avoid:

    • Red meat
    • Raw fruits and vegetables
    • Melons, radishes, horseradish, and turnips
    • Iron
    • Vitamin C (greater than 250mg)
    • NSAIDs

    Remember to follow the instructions carefully and ask your doctor any questions about what you can and cannot eat and when before you take your sample. You will be asked to wait to take your test if you have any condition that may cause blood or abnormal cells to be detected in your stool. These include:

    • Actively bleeding hemorrhoids
    • Menstrual cycle
    • History of polyps
    • Bleeding gums


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    Konrad, G. (2010, March). Dietary interventions for fecal occult blood test screening. Canadian Family Physician, 56(3), 229-238. Retrieved from PMC:

    Levine, H. (2016, June 27). How to prep for a fecal occult blood test. Retrieved from Consumer Reports:
    Mayo Clinic Staff. (2020, May 12). Fecal occult blood test. Retrieved from Mayo Clinic: