Fecal Occult Blood Tests (cont.)
Dennis Lee, MD
Dr. Lee was born in Shanghai, China, and received his college and medical training in the United States. He is fluent in English and three Chinese dialects. He graduated with chemistry departmental honors from Harvey Mudd College. He was appointed president of AOA society at UCLA School of Medicine. He underwent internal medicine residency and gastroenterology fellowship training at Cedars Sinai Medical Center.
In this Article
- 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 do the results of chemical fecal occult blood testing compare with immunologic fecal occult blood testing?
A chemical fecal occult blood test is inexpensive and easy, but it has several disadvantages.
- Substances in fruits and vegetables can mimic heme and cause chemical fecal occult blood tests to be falsely positive, that is, falsely abnormal. Therefore, it is necessary to restrict certain fruits and vegetables before and during the collection of stool samples.
- Unlike heme which can travel intact from the stomach or small intestine and into the stool, globin is destroyed in the small intestine. As a result, a positive chemical fecal occult blood test can be caused by bleeding anywhere in the stomach or intestines, but a positive immunologic fecal occult blood test only occurs when there is bleeding into the colon. Therefore, it is necessary to restrict red meat containing hemoglobin before and during the collection of stool samples or the heme from the ingested meat will cause a falsely positive test.
- Some drugs commonly cause small amounts of bleeding into the stomach or small intestine. Moreover, vitamin C and a few other drugs can cause an abnormal chemical fecal occult blood tests. Therefore, these drugs must be stopped before and during the collection of samples.
The immunologic fecal occult blood test has additional advantages over the chemical fecal occult blood test. First, it is more sensitive for blood. This means that given the same amount of blood in the stool, the immunologic fecal occult blood test will more frequently be abnormal. In other words, it will more frequently detect cancers and precancerous polyps. Second, it is more specific for blood. That is, there will be fewer abnormal tests due to interfering substances in the diet, and as a result, an abnormal immunologic fecal occult blood test will more commonly be due to cancer or a precancerous polyp. As a result, less follow-up testing (for example, colonoscopy) will be necessary to pursue a falsely abnormal fecal occult blood test.
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