what will a stool sample show
Frederick R. Jelovsek MD. When the doctor does a rectal exam and smears a stool sample on a card to test for blood in the stool, or gives you a paper to drop into the toilet to look for a color change, this is known as testing for occult gastrointestinal bleeding.
Colorectal cancer affects the large intestine (colon) and the rectum. There are three kinds of stool tests: Fecal occult blood test (FOBT). For this test, you put tiny samples of your stool on a special card or cloth and send it to a lab. The lab uses chemicals to find blood that you can't see with the naked eye. With some test kits, you can add the chemicals yourself at home. FOBT tests don't cost much.