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To screen for digestive tract bleeding, which may be an indicator of colon cancer
Stool samples are required. The sampling procedure depends on the choice of test, and the samples may be collected in the privacy of your home. (See the section "What is being tested?" below for more details.)
For the fecal immunochemical test (FIT), which is the preferred test, no preparation is necessary. For the guaiac-based FOBT (gFOBT, FOBT), you will be instructed to avoid certain medications and follow certain dietary restrictions for several days before collecting the stool samples.
Both the FIT and FOBT are designed to evaluate stool samples for hidden ("occult") blood, meaning blood that cannot be seen with the naked eye. Although there are several possible causes of blood in the stool, one important cause is the presence of polyps or cancers in the digestive tract.
Colon polyps are common as people age, but most polyps do not cause any health problems and are benign. However, a benign polyp can turn into a cancerous polyp and the cancer may even spread to other parts of the body (metastasize). If detected early, colon cancer can be successfully treated. It is therefore important to determine whether pre-cancerous or cancerous polyps are present.
Polyps are finger-like growths that protrude into the cavity (lumen) of the colon or the rectum. They can be fragile and bleed intermittently, such as when food waste brushes against them. This blood is mixed in with the stool and when the amounts are small, the blood can only be detected by tests for occult blood. This small amount of blood may be the first and sometimes the only sign of polyps or early colon cancer, making the stool-based tests valuable screening tools.
There are two principal methods for detecting occult blood in the stool. They are designed to detect hemoglobin, a molecule that is present in red blood cells. Hemoglobin has two essential parts: a chemical part called heme and a protein part called globin. Each method tests for a different part of the hemoglobin molecule.
How is the sample collected for testing?
Typically, you will be supplied with a kit to use at home. The collection method may vary based on the specific manufacturer of the kit. Follow the instructions that are provided with the kit. Multiple stool samples may have to be collected on different days and after the last sample is taken, all of the samples are brought to or mailed to your healthcare practitioner or laboratory.
Is any test preparation needed to ensure the quality of the sample?
For the FIT, there are no particular restrictions prior to testing. The test uses antibodies to detect only human blood from the lower digestive tract (colon).
For guaiac FOBTs, there are various restrictions:
Follow the instructions provided by your healthcare practitioner or included in test kit. Check with your healthcare provider before stopping any drugs to be certain that it is safe to do so.
The main use of tests for occult blood in the stool (FIT and FOBT) is to screen for colon polyps or early colon cancer in people with average risk. Blood in the stool may be the only sign of early colon cancer. If detected early, treatment can begin immediately, improving the chance of a cure. Persons with average risk include those without a family history of colon cancer and those without certain genetic diseases.
The FIT is preferred over the gFOBT because it is better at detecting cancer and it doesn't require dietary restrictions before testing, according to the U.S. Multi-Society Task Force (MSTF) on Colorectal Cancer.
For people with an increased or high risk of colon cancer, a colonoscopy is usually recommended for screening because it is the most accurate and thorough screen available. See the article on Colon Cancer or the health screening articles for Adults and Adults age 50 and older for additional details.
A secondary use of occult blood testing is to help determine if a person's anemia is due to blood loss in the gastrointestinal tract (GI tract), such as from a bleeding ulcer. If someone has signs and symptoms of anemia, such as fatigue, a low hemoglobin and hematocrit level, and/or unusually dark stools, a healthcare practitioner may order the FOBT. As noted above, the FOBT can detect blood from any part of the GI tract, while the FIT is more reliable in cases of bleeding from the lower part of the GI tract.
The American Cancer Society (ACS), the U.S. Multi-Society Task Force (MSTF) on Colorectal Cancer and U.S. Preventive Services Task Force (USPSTF) recommend yearly testing when you choose occult blood testing (FIT or FOBT) as the method of screening for colon cancer. The ACS advises that people of average risk begin screening at age 45. The MSTF and USPSTF advise that screening for colon cancer begin at age 50.
A healthcare practitioner may sometimes order an FOBT when someone has unexplained anemia that might be caused by bleeding in the digestive tract.
A negative fecal occult blood test means no blood was detected in the stool at the time of the test.
For the FIT, a positive result indicates abnormal bleeding in the lower digestive tract. While this bleeding could be caused by colon cancer, other possible causes include ulcers, polyps or hemorrhoids.
For the guaiac-based FOBT, a positive test result indicates that abnormal bleeding may be occurring somewhere in the digestive tract. This blood loss could be due colon cancer or to cancerous tumors. It could also be due to benign polyps, to ulcers, diverticulosis, inflammatory bowel disease, hemorrhoids or to blood swallowed due to bleeding gums or nosebleeds.
A positive result from either method requires follow-up testing. This usually involves direct imaging of the colon and rectum. A colonoscopy is typically recommended because it allows examination of the colon and the removal of any precancerous polyps and/or cancerous areas that are found.
Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples. Taking multiple samples on different days increases the chance of detecting bleeding that is intermittent. This is especially true for FOBT. The FIT is more sensitive, so a single sample is usually considered acceptable.
An occult blood test (FIT or FOBT) can give a false-negative result if the cancer or polyps do not bleed during the time the sample is taken. In the absence of polyps or colon cancer, a false-positive result may be obtained if you have bleeding from other sources, such as hemorrhoids, ulcers and inflammatory bowel disease.
Both FIT and FOBT can be performed in people with common hemoglobin disorders (hemoglobinopathies), such as sickle cell disease.
There is a combination stool DNA-FIT test available. The test detects occult blood plus certain genetic variations in cancer cells that are shed into the stool. Guidelines from the US Multi-Society Task Force on Colorectal Cancer recommend the stool DNA-FIT as a tier 2 test. It is a tier 2 test because it has some disadvantages compared to the recommended tier 1 tests, colonoscopy, and annual FIT. (See the article on Colon Cancer for additional details on screening options.) If you choose the DNA-FIT method of screening, it is advised that you repeat it every 3 years, as long as the results are negative.
Yes. There are imaging tests that may be used for the detection of precancerous polyps and colon cancer. One of these procedures may be chosen instead of screening annually with a fecal occult blood test:
Several home FOBT tests are available and while they are fairly sensitive, your healthcare practitioner may prefer a test performed by laboratories. Laboratory professionals are more experienced than most people in interpreting results and are more familiar with sources of false-negative and false-positive results. Read the article With Home Testing, Consumers Take Charge of Their Health to find out more about the pros and cons of home tests.
An FOBT or FIT may be preceded or followed by a digital rectal exam to detect any growths that may be present in the rectum. The DRE is primarily performed to examine the prostate gland in men but also allows for the examination of the lower rectum, pelvis, and belly. Most colon cancers, however, are beyond the detection range of a DRE.
It is recommended that a positive FOBT or FIT be followed by colonoscopy, which enables the healthcare practitioner to examine the entire colon and to identify and remove any precancerous polyps or cancerous areas.
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