The fecal occult blood test (FOBT) is used to find occult blood (blood that can't be seen with
the naked eye) in feces. The idea behind this test is that blood vessels at the surface of larger
colorectal polyps or cancers are often fragile and easily damaged by the passage of feces. The
damaged vessels usually release a small amount of blood into the feces, but only rarely is
there enough bleeding for blood to be visible in the stool.