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:
Differential Diagnosis of Occult Gastrointestinal Bleeding
- carcinoma of any site
- large (more than 1.5 cm) adenomatous polyp
- erosive esophagitis
- ulcer of any site
- linear cuts in a hiatus hernia
- erosive gastritis
- celiac disease
- ulcerative colitis
- Crohn's disease (regional enteritis)
- nonspecific colitis
- unexplained cecal ulcer
- vascular ectasia (blood vessel inflammation)
- liver vein hypertension
- gastrointestinal varicose veins (varices)
- other blood vessel abnormalities
- tuberculous enterocolitis
- coughing blood from lungs and swallowing it
- nosebleeds and mouth cavity bleeding
- long distance running
- falsified causes
I have heard iron supplementation and sometimes aspirin can cause a false positive test. Is that true?
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. Some studies of patients on anticoagulants who had positive fecal occult blood tests show over 90% causes from major gastrointestinal conditions rather than just due to the anticoagulants.
There are occasionally other causes of false positive fecal occult blood tests that are from ingested substances. Any hemoglobin/blood in red meat can affect the test as well as some vegetable enzymes on occasion
Is there a relationship between iron deficiency anemia and gastrointestinal bleeding?
Iron deficiency anemia is usually diagnosed by a hemoglobin value of less than 12 g/dl and a serum ferritin level of less than 45 ug/l. In the U.S., 5-11% of women have iron deficiency anemia, often on a basis of menstrual bleeding problems. However, chronic occult gastrointestinal bleeding is also a frequent cause of iron deficiency. Anytime a woman is diagnosed with anemia and it turns out to be of the iron deficient form, an investigation should be made to make sure the blood loss is not from somewhere other than menses. At a minimum, a test for fecal occult blood should be performed. As much as 12% of iron deficiency anemia in premenopausal women may be due to serious gastrointestinal anomalies.
What happens if they can't find the cause for a positive fecal occult blood test?
When the work up to evaluate occult fecal blood is completed (see colon cancer screening article below), about 5% of patients with gastrointestinal bl
eeding will not have a source for their bleeding found. In this case some additional studies may be needed and sometimes "oscopy" studies may need to be repeated with different or more experienced gastorenteroscopists.
A radioactive technetium-99m scan may be performed which can detect as small a blood loss as 0.1 ml per minute. It is useful only to confirm that there is GI bleeding and where in the GI tract,in general, the bleeding is occurring. It still does not diagnose the cause. Enteroscopy is another study that may be used to look at the small intestine which cannot be seen with gastroscopy (looking in the stomach) and colonoscopy (looking up the colon). Sometimes even after all these studies, a source of the bleeding may be found. When that happens the best that can be done is just to have close follow up and occasionally repeat any of the studies as indicated by symptoms.
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