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Medical Definition of Occult blood
1. Blood that is in the faeces or vomitus that is not obvious by general inspection. Used as a diagnostic screening test for colon cancer. Tests for occult blood are generally colourimetric. Stool sample will turn blue when a particular chemical reagent is mixed with the specimen on test paper. Kits for measuring occult blood in the stools are now available at most drug stores. (27 Sep 1997)