Fecal occult blood test is of great value in the diagnosis of gastrointestinal bleeding and is often used as a screening index for the early diagnosis of gastrointestinal malignancies. The physiological blood loss of gastrointestinal tract in 24h of normal people is about 0.6ml, and it is generally difficult to detect by high-sensitivity chemical occult blood test (which can detect gastrointestinal bleeding more than 1ml). The use of more sensitive and specific immunological methods can sometimes appear false positive, so it is worth noting. Positive: intermittently positive in ulcerative bleeding of the digestive tract; Gastrointestinal cancer is persistently positive, so it can be used as a differentiator between benign and malignant bleeding. Positive also seen in intestinal tuberculosis, ulcerative colitis, colon polyps, hookworm disease, renal bleeding syndrome.