Fecal Testing

Polyps contain fragile blood vessels that can be ruptured by the passage of feces. When this happens, blood is incorporated into the stool.  A common screening test for colorectal cancer is called the fecal occult blood test (FOBT). This screening test can be performed at home. Briefly, the process involves collecting stool samples onto a special collection card from three separate bowel movements. This card is then tested in a laboratory for trace amounts of blood which cannot be seen by the naked eye. If blood is detected in any of the stool samples, a colonoscopy should be performed to determine the cause of the bleeding. 

A limitation of the FOBT is that early stage colorectal cancer does not always bleed.  As a result, early cancers can be missed using this screening approach.  A second limitation of this test is that many people find it unpleasant to collect multiple stool samples.  The fecal immunochemical test (FIT) is similar to the FOBT but it is more specific. While the FOBT can detect animal blood (from meat) in the stools, the FIT is specific for human blood that originates from the colon or rectum.  Because of this, the FIT tends to be simpler, requiring less preparation than the standard FOBT.

Unlike both the FOBT and the FIT which identify blood in stool, a third type of fecal-based screening known as the stool DNA test (sDNA or the fecal DNA test) detects abnormalities in the DNA of cells from tumours or polyps that can be found in a stool sample.  The test requires collecting an entire stool specimen. 

The FOBT is the most common type of fecal-based screening.  For all fecal testing, a follow-up colonoscopy is required if the results are positive.