Fecal occult blood is small amounts of blood hidden in the feces, notes Mayo Clinic. Tests for fecal blood are done to see if there is blood loss in the gastrointestinal tract, which stretches from the mouth to the colon, according to Wikipedia.
An individual who tests positive for fecal occult blood may have a peptic ulcer, polyps, gastric cancer, colorectal cancer or another type of malignancy, according to Wikipedia.
Usually, the only way to detect minute traces of blood in the feces is with the special chemicals used during a fecal occult blood test, notes Mayo Clinic. A fecal blood test is only able to detect whether or not there is blood in the feces. It can't determine exactly where the bleeding is coming from.