Definitions

fecal occult test

Fecal occult blood

Faecal occult blood is a term for blood present in the faeces that is not visibly apparent. In medicine, a faecal occult blood test is a check for hidden (occult) blood in the stool (feces). Conventional faecal occult blood tests look for heme. Newer, modern tests look for globin.

Uses

Faecal occult blood testing - as its name implies - can provide 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 faecal 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 of a population may reduce the mortality associated with colon cancer by a third, depending on the incidence of gastrointestinal cancer in that population. It is not always cost effective to screen a large population.

If colon cancer is suspected in an individual (such as in someone with an unexplained anaemia) faecal occult blood tests are typically not warranted. If a doctor suspects colon cancer, more rigorous investigation is necessary, whether or not the test is positive.

Methodology

For a simple, traditional test, foods to avoid include red meat (the blood it contains can turn the test positive), radishes, turnips, cabbage, cauliflower, horseradish, uncooked broccoli, and cantaloupe (all of which contain a chemical that can turn the test positive), and citrus fruits and vitamin C supplements (which can turn the test falsely negative).

Checking for hidden (occult) blood in the stool can be done at home. Testing kits are available at pharmacies without a prescription, or a health professional may order a testing kit for use at home. If a home fecal occult blood test detects blood in the stool, a health professional should be contacted.

There are three methods for measuring blood in feces:

One method: Stool guaiac test 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 color. 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.

Sensitivity

The common, traditional guaiac-based fecal occult blood test usually picks up a daily blood loss of about 10 ml (about two teaspoonfuls). The sensitivity of a single FOBT has been quoted at 30%, but if 3 tests are done (as is standard), the sensitivity rises to 92%. "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 guaiac such as a heme-porphyrin test or an 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... It does not detect blood from the stomach and upper small intestine so it is much more specific for bleeding from the colon or lower gastrointestinal tract.

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.

The stool-based DNA test, PreGen-Plus was capable of detecting several stages of colorectal cancer in otherwise healthy adults, most importantly in its early stages, when cancer treatment is most effective.

Results

The DNA based PreGen-Plus's results were reported by the New England Journal of Medicine see their website for more.

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:

See also

References

External links

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