Fecal occult blood test (FOBT)

The iFOBT (immunochemical fecal occult blood test), also referred to as the FIT (fecal immunochemical 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.

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 iFOBT 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, which includes the latest recommendations from Quebec’s ministry of health and social services (MSSS).

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