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Fecal occult blood test (FOBT)

Raymond Lepage, PhD
Raymond Lepage, PhD
Senior Scientific Advisor

What is the purpose of this test?

At Biron, we refer to this test as the fecal immunochemical test (FIT), while in hospitals and CLSCs, it is commonly called the fecal occult blood test (FOBT). Whatever its name, checking stool samples for blood using an immunological technique is an extremely useful test for detecting colon cancer.

The test is based on the fact that large polyps, tumours (also called adenomas) and colon cancers release very small amounts of blood into the intestines, and this blood ends up in the stools. These amounts are generally too small to be detected with the naked eye, hence the term “occult blood.” In theory, the larger or more advanced a 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 enough on the day you collect your stool sample for the test to come up positive. More on that later.

Diagnostic or screening test?

A fecal occult blood test is not used for diagnosis, but for screening purposes. Because several other situations can lead to blood in the stool, a positive FOBT does not necessarily mean that cancer is present. Hemorrhoids, anal fissures, constipation or inflammatory bowel disease can also cause blood in the stool. If the blood test is positive, it is estimated that there is a 25% (one in four) chance that the bleeding is due to a large polyp or colorectal tumour, and about a 2.5% (one in forty) chance of cancer being present. The only way to find out is to have a colonoscopy, a test that involves inserting a tube into the rectum and “pushing” it as far as possible into the large intestine to look for a tumour or cancer, while taking the opportunity to remove polyps or tumours that could eventually become problematic. There is also a procedure performed by radiologists called a “virtual colonoscopy” which is equally effective at detecting polyps, tumours, cancers and other abnormalities, but does not permit their removal at the same time. Unfortunately, a colonoscopy, whether regular or virtual, remains expensive, difficult to access, somewhat uncomfortable and a procedure that carries a risk of complications. Therefore, unless there is a particular risk factor, it is more convenient to have a fecal occult blood test first, then follow up with a colonoscopy if the blood test comes up positive. This is the principle behind all colon cancer screening programs, including those in Quebec.

An FOBT is the best screening test for a population that is said to be at “moderate risk” of developing colon cancer. A “moderate risk” population includes men and women aged 50 and over who do not have additional risk factors such as first-degree relatives (father, mother, brother or sister) diagnosed with colorectal cancer, a previous diagnosis of colon cancer themselves or an illness with many polyps. In such cases, a colonoscopy is essentially inevitable, therefore the step of performing an FOBT can effectively be skipped.

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) or preferably every year (U.S. recommendations). Some Canadian provinces (Newfoundland, British Columbia and Nova Scotia) even require the collection of two consecutive specimens to improve cancer detection.

Colon 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 colon cancer is detected, the easier it is to cure. A fecal blood test remains a simple, affordable and very effective technique for screening.

If you are between the ages of 50 and 75 and think you are part of the “moderate risk” population, you are encouraged to follow the recommendations of your doctor concerning fecal blood testing. It could save your life!

If you have any questions or need more information, please don’t hesitate to call our customer service number at 1 833 590-2715.