Overview
A fecal occult blood test looks for blood in a sample of stool. It can find tiny amounts of blood that can't be seen just by looking at the stool. The medical term for this hidden blood is occult blood. Fecal occult blood test is often shortened to FOBT.
A fecal occult blood test is one option for colon cancer screening in people who don't have any symptoms. Occult blood in the stool may be a sign of cancer or polyps in the colon or rectum. Polyps are growths of cells that aren't cancers but could become cancers. Not all cancers or polyps bleed.
If a fecal occult blood test finds blood, more tests may be needed to find the source of the bleeding. The fecal occult blood test only can detect if blood is present or not present. It can't find the source of the bleeding.
A fecal occult blood test shouldn't be used if you have colon cancer symptoms. These symptoms may include blood in your stool or in the toilet, pain in your abdomen, and a change in bowel habits. This test also isn't used in people with a high risk of colon cancer. This includes those who have a history of colon polyps and those with a close family member who has had colon cancer. Talk with your healthcare team about your symptoms and health history if you are considering colon cancer screening.
Why it's done
A fecal occult blood test is used to look for blood in a sample of stool. It's one option for colon cancer screening. It may be used if you have an average risk of colon cancer and no symptoms. The fecal occult blood test is typically done every year.
The fecal occult blood test is one of several colon cancer screening tests available. Talk with your healthcare team about which tests might be right for you. The fecal occult blood test is a simple test that requires little or no preparation. Some people prefer this test over the other screening tests because it can be done at home. It doesn't require missing work for a medical appointment. Others may choose this test because it often is less expensive than other tests.
Risks
Risks and limitations of the fecal occult blood test include:
The test isn't always accurate
A fecal occult blood test looks for hidden blood. If no blood is found, the test result is negative. But this could be a false-negative result if you have cancer or polyps that don't bleed. The result also might be a false-negative if blood is present but is not detected. That's why healthcare professionals recommend the test be done every year.
If the test result is positive, that means the test found blood. But it's a false-positive result if the bleeding is due to something other than cancer or polyps. The blood could be coming from a stomach ulcer or a hemorrhoid. It even could be swallowed from your mouth or your nose.
The test may lead to more testing
If your fecal occult blood test result is positive, your healthcare professional may recommend a test to examine the inside of your colon. The test is called a colonoscopy.
How you prepare
To prepare for a fecal occult blood test, you may need to change what you eat and what medicines you take. Various foods, supplements and medicines can affect results of some fecal occult blood tests. The tests may show that blood is present when it isn't, causing a false-positive. Or they may miss blood that's there, causing a false-negative.
Before the test your healthcare professional may ask you to avoid:
- Certain fruits and vegetables.
- Rare red meat.
- Certain vitamin supplements, such as vitamin C and iron.
- Pain relievers, such as aspirin and ibuprofen (Advil, Motrin IB and others).
Not all fecal occult blood tests require these preparations. Follow your healthcare professional's instructions.
What you can expect
What you can expect when taking a fecal occult blood test depends on the type of test you take. Each type collects and tests stool samples differently. For best results, follow the instructions that come with your test kit.
You may receive a fecal occult blood test kit from your healthcare professional. Or your health professional may arrange to have the kit sent to you by mail. The kit typically includes everything you need to complete the test. The instructions may explain how to catch a bowel movement in the toilet bowl, collect and place a stool sample on a card or in a container, and send the sample to a lab for testing.
Fecal immunochemical test
One type of fecal occult blood test is a fecal immunochemical test, also called FIT. For this test, you collect a sample from one bowel movement. The test kit may include a stick or other device that you use to collect the sample. You place the sample in a container that comes with the test kit. Then you return the container to a lab for testing, often by mail.
Sometimes fecal immunochemical test kits are sold in drugstores. They don't require a prescription. Some kits give the results in a few minutes. Other kits instruct you to send your sample to a lab. You report the results to your healthcare professional.
The fecal immunochemical test is often the preferred fecal occult blood test because it's more sensitive. Also, you don't need to follow any diet restrictions ahead of time.
Guaiac fecal occult blood test
Another type of fecal occult blood test is a guaiac fecal occult blood test, also called gFOBT. For this test, you typically collect samples from three bowel movements on separate days. The samples from each bowel movement are smeared onto a card provided in the test kit. You typically send the card to a lab for testing.
Some guaiac-based tests use flushable pads that change color if there is blood in the stool. These are sometimes sold in drugstores and don't require a prescription. For this type of test, you place the pad in the toilet bowl after a bowel movement. The pad changes color when blood is present. You report the results to your healthcare professional.
Results
Your healthcare professional may review the results of the fecal occult blood test and then share them with you. Ask when you can expect your results. Results may include:
- Negative result. A fecal occult blood test is negative if no blood is found in your stool. If you have an average risk of colon cancer, your healthcare professional may recommend repeating the test yearly.
- Positive result. A fecal occult blood test is positive if blood is found in your stool. Your healthcare professional may recommend a colonoscopy to find the source of the bleeding.