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 FOBT and immunochemical FOBT are two types of FOBTs. Guaiac FOBT uses a chemical substance called guaiac to check for blood in the stool. Immunochemical FOBT uses an antibody to check for blood in the stool. Also called fecal occult blood test.