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Specialist Advice — 14 minutes

Fecal occult blood test (FOBT)

Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer

What is the purpose of this test?

The FIT (fecal immunochemical test), also referred to as the iFOBT (immunochemical fecal occult blood test), looks for blood in the stool using an immunochemical technique, which is extremely useful for detecting colorectal cancer.

The test is based on the fact that large polyps, tumours (also called “adenomas”) and cancers discharge very small amounts of blood into the intestine, which are then passed in the stool. These amounts are generally too small to be detected with the naked eye, hence the term “occult blood.” In theory, the larger and more advanced the polyp, tumour or cancer, the greater the amount of blood in the stool. However, this is not always the case. Such bleeding tends to be irregular and it is possible that a tumour, even a large one, will not have bled sufficiently on the day the stool sample is taken for the test to be positive. More on that later.

Is it a diagnostic or screening test?

A fecal occult blood test is not used for diagnosis, but for screening purposes. Because a number of other situations can lead to blood in the stool, a positive FIT does not necessarily mean that cancer is present. Hemorrhoids, anal fissures, constipation or inflammatory bowel disease can also cause blood in the stool. In Quebec, out of every 1,000 people who get tested, 36 will have blood in their stool (3.6%). Of these 36 people,

  • 4 will have colorectal cancer;
  • 17 will have one or more polyps;
  • 15 will have neither polyps nor cancer.

The diagnostic test for colorectal cancer is optical or virtual colonoscopy. Optical colonoscopy involves inserting a camera-equipped tube into the rectum and “pushing” it as far as possible through the large intestine in search of a tumour or cancer, taking the opportunity along the way to remove any polyps or tumours that may become problematic. Virtual colonoscopy, on the other hand, is a computed tomography (CT) scan performed by a radiologist in order to detect polyps, tumours, cancers and other abnormalities, but does not remove them. Colonoscopy, whether optical or virtual, is an expensive and difficult-to-access test which can cause discomfort and lead to complications. This is why Quebec’s colorectal cancer screening program now recommends that symptom-free people with no other risk factors undergo an occult blood test first and that they get a colonoscopy only if the initial test is positive.

A fecal blood test is the best screening tool for a population considered at “average” or “slightly increased” risk of developing colorectal cancer. To find out what your risk factor is, we invite you to complete the colorectal cancer screening questionnaire (FIT test), which includes the latest recommendations from Quebec’s ministry of health and social services (MSSS).

What does a negative result mean?

Because tumours do not release blood on a continuous basis, a negative test does not completely eliminate the possibility of a tumour or cancer being present. This is why the test should be repeated at least every two years (MSSS program).

Colorectal cancer is the second-deadliest form of cancer in men, after lung cancer. In women, it ranks third behind breast cancer and lung cancer. It is well known that the earlier colorectal cancer is detected, the easier it is to cure. A fecal blood test remains a simple, affordable and very effective technique for screening.

Would you like to take part in the FIT screening program to prevent colorectal cancer? If so, don’t hesitate to complete the questionnaire to find out if you’re eligible for the program. It could save your life!

You have concerns regarding colorectal cancer? Fill out our short questionnaire to confirm your eligibility for the program and to order your screening kit.

Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer
For about 50 years, Raymond Lepage worked as a clinical biochemist in charge of public and private laboratories. An associate clinical professor at the Faculty of Medicine of the Université de Montréal and an associate professor at the Université de Sherbrooke, he has also been a consultant, researcher, legal expert and conference speaker. He has authored or co-authored more than 100 publications for scientific conferences and journals, and now devotes part of his semi-retirement to popularizing science.