The guiac-based test will turn positive from bleeding anywhere in the GI tract but it will also turn positive if you have had a recent meal of red meat. Other dietary factors and even stool transit time can affect the test outcome. Any process that causes an increased amount of bleeding in the GI tract will turn the test positive. This could be a cancer, but it also could come from an ulcer, bacterial infection, blood vessel malformation, parasite infections or even nosebleeds. I have heard iron supplementation and sometimes aspirin can cause a false positive test.
Many people believe that oral iron medications cause a false positive fecal blood test.
They really do not. The only problem is that it may confuse someone in reading the test because the dark green or black appearance of iron in the stool may may be mistaken for the blue color typical of a positive guiac test. Another misconception is that aspirin or anticoagulants often cause GI tract bleeding. While it is possible that this happens on some occasions, it is actually infrequent, i.e., most patients on anticoagulants or aspirin do not have guiac detectable fecal blood levels. There are occasionally other causes of false positive fecal occult blood tests that are from ingested substances.