The fecal occult blood test (FOBT) is a lab test used to check stool samples for hidden (occult) blood.Occult blood in the stool may indicate colon cancer or polyps in the colon or rectum — though not all cancers or polyps bleed.ypically, occult blood is passed in such small amounts that it can be detected only through the chemicals used in a fecal occult blood test. If blood is detected through a fecal occult blood test, additional tests may be needed to determine the source of the bleeding.