Occult gastrointestinal bleeding is loss of blood into the digestive tract that is not apparent to the patient or physician by physical examination. It is detected by examination of the stool for chemical evidence of blood by laboratory techniques or by the observation of iron deficiency. The presence of occult blood is important because it may indicate otherwise asymptomatic gastrointestinal neoplasia, assist with the evaluation of gastrointestinal symptoms in the absence of visible bleeding, and point toward a digestive tract source of blood loss in the patient with iron-deficiency anemia.