Fecal occult blood
From Wikipedia, the free encyclopedia
| ICD-10 | K92.1 |
|---|---|
| ICD-9 | 792.1 |
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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[edit] Uses
Fecal 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 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 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) fecal occult blood tests are usually not warranted. If a doctor suspects colon cancer, more rigorous investigation is necessary, whether or not the test is positive.
[edit] Methodology
There are three methods for measuring blood in feces:
- (Hemoccult® or Instaccult®). This method can reduce death from colorectal cancer.
- Fecal porphyrin quantification (Hemoquant®) - high false positive rate.
- Immunochemical fecal occult blood tests (HemeSelect®) or (QuickVue® iFOB) - more specific.
- Fecal DNA test (PreGen-Plus®) is more sensitive than fecal occult blood in one study (51.6 % vs. 12.9%)[1]
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 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.
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. Another test called QuickVue® iFOB test requires only one specimen, and becauses it is specific to human hemoglobin, patients are not required to adhere to strict dietary or medication restrictions. The result is a more patient friendly test that is easier to complete.
[edit] 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, and most importantly in its' early stage, the easiest and most effective to treat, stage of colorectal cancer.
[edit] 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:
- 2-10%: cancer (colorectal cancer, gastric cancer)
- 20-30% adenoma or polyps
- Bleeding peptic ulcer
- Angiodysplasia of the colon
[edit] References
- ^ Imperiale T, Ransohoff D, Itzkowitz S, Turnbull B, Ross M (2004). "Fecal DNA versus fecal occult blood for colorectal-cancer screening in an average-risk population". N Engl J Med 351 (26): 2704-14. PMID 15616205.
[edit] Sources
- [1] notes on fecal occult blood testing.
- [2] for the DNA based PreGen-Plus® information sheet from The New England Journal of Medicine.
