Fecal occult blood
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Fecal occult blood is a term for blood present in the feces that is not visibly apparent. In medicine, a fecal occult blood test is a check for hidden (occult) blood in the stool (feces). Conventional fecal occult blood tests look for heme. Newer, modern tests look for globin.
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Uses
Fecal occult blood testing - as its name implies - can prove clues as to subtle blood loss in the gastrointestinal tract, anywhere from the mouth to the colon. Positive tests ("positive stool") warrant further investigation for peptic ulcers or a malignancy (such as colorectal cancer or gastric cancer).
In the event of a positive fecal occult blood test, the next step in the workup is a form of visualization of the gastrointestinal tract (ie: endoscopy, colonoscopy, virtual colonoscopy).
Annual testing may reduce the mortality associated with colon cancer by a third.
Methodology
There are three methods for measuring heme in feces:
- Guaiac-based test (Hemoccult® or Instaccult®)
- Fecal porphyrin quantification (Hemoquant®) - high false positive rate.
- Immunochemical fecal occult blood tests (HemeSelect®) - more specific.
One method: the test involves smearing some feces onto some absorbent paper that has been treated with a chemical. Hydrogen peroxide is dropped onto the paper; if trace amounts of blood are present, the paper will change colour. This method works as hemoglobin has a peroxidase-like effect, rapidly breaking down hydrogen peroxide.
Since 2001, there are a new class of occult blood tests called Fecal Immunochemical Tests.
These tests detect the globin in feces rather than heme. By detecting globin the tests are both more sensitive and specific for lower gastrointestinal bleeding.
One test, called Insure, is designed to address patient ease of use by using a brush, not a wooden stick, to sample stools while in the toilet bowl. Using these tests there is no direct fecal handling and there is no need for changing diet or medication to perform the test. Finally the newer tests are more sensitive and so require only two samples rather than the traditional three day's testing.
Interpretation
The test is often false-positive (i.e. there is no source of bleeding). This is often due to recent ingestion of under-cooked meats, and a patient is generally advised to keep a meat-free diet for several days before handing in the feces sample.
False negatives may result if the patient has been taking vitamin C supplements.
The test is more sensitive if the sample is hydrated before testing. However, the specificity is decreased in this method.
Newer, more sophisticated assays called Fecal Immunochemical Tests have been developed to address most of the deficiencies of traditional guaiac based fecal occult blood tests.
Results
An estimated 1-5% of the tested population have a positive fecal occult blood test. Of those, about 2-10% have cancer, while 20-30% have adenomas.
Causes for a positive test are:
- 2-10%: cancer (colorectal cancer, gastric cancer)
- 20-30% adenoma or polyps
- Bleeding peptic ulcer
- Angiodysplasia of the colon
Sources
- Family practice (http://www.fpnotebook.com/GI144.htm) notes on fecal occult blood testing.