What is a fecal occult blood test?
Fecal occult blood testing (FOBT) is testing that is performed on samples of stool in order to detect occult blood (blood that is not visible to the naked eye) in otherwise normal-colored stool. Fecal occult blood usually is a result of slow (often intermittent) bleeding from inside the upper or lower gastrointestinal tract. The slow bleed does not change the color of the stool or result in visible bright red blood.
Therefore, the blood is found only by testing the stool for blood in the laboratory. Occult bleeding has many of the same causes as other forms of more rapid gastrointestinal bleeding, such as rectal bleeding (passage of red blood and/or blood clots rectally) and melena (black tarry stool as a result of bleeding from the upper intestines such as ulcers). Please read the Rectal Bleeding article for a more detailed explanation of different patterns of gastrointestinal bleeding.
Why is fecal occult blood testing done?
A fecal occult blood test is done primarily to detect colon cancer in people without intestinal symptoms. Cancers of the colon are common and frequently produce fecal occult blood long before they cause other symptoms such as abdominal pain, rectal bleeding, or changes in bowel habits. In addition, some precursors of colon cancer, specifically, some types of colon polyps, also may bleed slowly and cause fecal occult blood. By finding the cancers early through fecal occult blood testing when the cancers are small and before they metastasize (spread to distant organs), patients may be cured of their cancers by removing them surgically. By removing precancerous polyps that are found through fecal occult blood testing, cancer may be prevented.
How is a fecal blood test performed?
For fecal occult blood testing, several (usually three) samples of stool are collected for testing. The reason for testing multiple samples is that bleeding from cancers and polyps often is intermittent and only one of the samples may show blood.
There are two types of fecal occult blood testing, 1) chemical and 2) immunologic.
- Chemical testing: For chemical testing, a solution containing the chemical guaiac and an oxidizing chemical is used. If blood is present in the sample of stool, the mixing of the solution with blood causes the guaiac to turn visibly blue. The blue color is caused by the interaction (promoted by the oxidizing agent) of the heme portion of the hemoglobin molecule, the oxygen carrying molecule in red blood cells, and the guaiac.
- Immunologic testing: For immunologic testing, a sample of stool is mixed with a solution that contains an antibody to globin, the protein part of the hemoglobin molecule. The antibody is combined with a small amount of gold. When the antibody/gold complex binds to the globin in stool, the antibody/gold/globin complex settles out of the solution as a visible line on the test strip.
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.
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.