What Does a Positive Fecal Occult Blood Test Mean?
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. Occult means you do not know there is blood in the stool, i.e., you cannot see bright red recognizable blood like you might see if you had bleeding from hemorrhoids. Testing for fecal occult blood is commonly used as a cancer screen for colon cancer. A positive test can mean other conditions however. A review article, Rockey DC: Occult gastrointestinal bleeding. N Engl J Med 1999;341:38-46 addresses some questions you might have if you turned out to have a positive test.
How much bleeding in the gastrointestinal tract is needed to turn a test positive?
There are two answers to this question. It depends upon which specific chemical method is used to detect the blood and also where in the gastrointestinal (GI) tract the bleeding occurs.
The most commonly used method is the guaiac-based fecal occult blood test. It usually picks up about a daily blood loss of about 10 ml (about two teaspoonfuls). Normally, there is only about 0.5 to 1.5 ml of blood a day that escapes blood vessels into the stool each day. There are more sensitive tests than the guiac such as a heme-porphyrin test or a immunochemical test, but the former test is not used much due to the high false positive rate. The latter test is very sensitive -- it picks up as little as 0.3 ml -- but it needs to be processed in a laboratory so it is much less likely to be used as a screening test. It does not detect blood from from the stomach and upper small intestine so it is much more specific for bleeding from the colon or lower gastrointestinal tract.
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.
What are some of the causes other than colon cancer that can show a positive fecal occult blood test?
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. The differential diagnosis is: