A test that checks for occult (hidden) blood in the stool. Small samples of stool are placed on special cards and sent to a doctor or laboratory for testing. Blood in the stool may be a sign of colorectal cancer or other problems, such as polyps, ulcers, or hemorrhoids. Guaiac fecal occult blood test and immunochemical fecal occult blood test are two types of fecal occult blood tests. Guaiac fecal occult blood test uses a chemical substance called guaiac to check for blood in the stool. Immunochemical fecal occult blood test uses an antibody to check for blood in the stool. Also called FOBT.