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occult blood (s) (noun)
, occult bloods (pl)
The bodily fluid that exists in various products of the body which is too difficult to see or to find: There are situations in which blood exists in such small quantities; such as, in feces, that the only way to find any occult blood
is by microscopic examinations or by chemical procedures.
Sometimes, determining whether a patient has colorectal (colon and rectum) cancer is done by testing the occult blood in the feces of those who are suffering from some ailment but who do not show any obvious symptoms or indications of what is actually causing their discomfort.