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- What is a fecal occult blood test?
- Why is fecal occult blood testing done?
- How is a fecal blood test performed?
- How do the results of chemical fecal occult blood testing compare with immunologic fecal occult blood testing?
- How effective are fecal occult blood tests?
- How should fecal occult blood tests be used?
How effective are fecal occult blood tests?
The true bottom line for a fecal occult blood test is how many cancers or precancerous polyps can be detected and, ultimately, how many cancer-related deaths can be prevented by fecal occult blood tests. Large studies with chemical fecal occult blood tests have demonstrated at least a 15%-20% reduction in mortality due to colon cancer. Similar studies are now underway with immunologic fecal occult blood tests, and, based on factors discussed previously, the results are expected to be better. Nevertheless, the results of these ongoing studies are necessary to demonstrate what the bottom line truly is for immunologic fecal occult blood tests.
How should fecal occult blood tests be used?
Fecal occult blood tests offer an important screening modality for colon cancer, but, because it detects only a minority of cancers and precancerous polyps, by itself it is not sufficient. It needs to be complemented by endoscopic examination of the colon, either sigmoidoscopy and/or colonoscopy.
Recommendations for the age at which fecal occult blood tests and endoscopic examinations should be initiated, and the frequency with which they should be repeated depend on the risk for colon cancer. Patients who are older, have relatives with colon cancer, have had polyps previously, or have diseases that are associated with a higher risk for colon cancer may have testing that begins earlier and is done more frequently.
The use of fecal occult blood tests has only one downside. Tests can be falsely positive, that is, tests that show blood in patients who don't have colon cancer or premalignant polyps. Every test showing blood requires the performance of colonoscopy, an expensive procedure that carries a small but definite risk of serious complications. The more frequently tests are falsely positive, the more frequently unnecessary colonoscopies are performed. If studies underway (discussed previously) demonstrate an increased ability to diagnose cancer and colon polyps with a reduced frequency of these falsely positive tests, there will be clear advantages for immunologic over chemical fecal occult blood tests. For this reason, immunologic fecal occult blood tests have advantages over chemical fecal occult blood tests since it has fewer false positive tests.
It is important to remember that having occult blood in the stool does not automatically imply that one has colon cancer or a polyp. There are many causes of occult blood in the stomach, small intestine, or colon, most of which are benign. Some examples include bleeding from use of aspirin or other nonsteroidal anti-inflammatory drugs (for example, ibuprofen), ulcers, esophagitis, colitis, and intestinal angiodysplasias (weakened clusters of blood vessels on the surface of intestinal linings). Therefore, patients with stools containing occult blood should be evaluated by physicians to exclude cancers and polyps, as well as to identify other causes of intestinal bleeding.
Medically reviewed by Avrom Simon, MD; Board Certified Preventative Medicine with Subspecialty in Occupational Medicine
"Tests for screening for colorectal cancer: Stool tests, radiologic imaging and endoscopy"
UptoDate. Updated Aug 23, 2016.