The fecal occult blood test (FOBT) checks 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. If blood is detected through a fecal occult blood test, additional tests such as a colonoscopy may help determine the source of the bleeding. The fecal occult blood test can only detect the presence or absence of blood — it can't determine what's causing the bleeding.