A: occult blood means that when there is a small amount of bleeding in the digestive tract, red can't be observed with the naked eye and red blood cells can't be found under the microscope, because blood is decomposed by digestive juice. At present, monoclonal antibody technology is widely used in OBT, which is not affected by animal hemoglobin. Monoclonal antibody technique is more sensitive and effective in judging lower gastrointestinal bleeding, while about 40% patients with upper gastrointestinal bleeding have false negative. For samples with obvious black stool or "cypress oil stool", it is necessary to use chemical methods or dilute the sample for determination.

The normal reference value is negative.

The clinical significance of occult blood positive can be seen as follows: ① digestive tract ulcer, mostly intermittent; ② Gastrointestinal tumor, persistent or intermittent; ③ Others, any causes or diseases that lead to gastrointestinal bleeding, such as drugs, intestinal tuberculosis, Crohn's disease, etc.