The fecal occult blood test (FOBT) is a lab test used to check stool samples for hidden (occult) blood.
Occult blood in the stool may indicate colon cancer or polyps in the colon or rectum — though not all cancers or polyps bleed.
Typically, occult blood is passed in such small amounts that it can be detected only through the chemicals used in a fecal occult blood test.
If blood is detected through a fecal occult blood test, additional tests may be needed to determine the source of the bleeding. The fecal occult blood test can only detect the presence or absence of blood — it doesn't indicate potential sources of bleeding.
Your doctor may recommend a fecal occult blood test to:
- Screen for colon cancer. If you're age 50 or older and at average risk of colon cancer, your doctor may recommend a fecal occult blood test every year to screen for colon cancer. In addition, however, you may need other screening tests that allow the doctor to examine the colon directly.
- Evaluate possible causes of unexplained anemia. Anemia is a condition in which there aren't enough healthy red blood cells to carry adequate oxygen to your tissues. Sometimes a fecal occult blood test is used to determine whether bleeding in your digestive tract — such as a bleeding ulcer — is contributing to anemia.
Risks and limitations of the fecal occult blood test include:
The test isn't always accurate. Your fecal occult blood test could show a negative test result when cancer is present (false-negative result) if your cancer or polyps don't bleed.
Your test could show a positive result when you have no cancer (false-positive result) if you have bleeding from other sources, such as a stomach ulcer, hemorrhoid, or even blood swallowed from your mouth or your nose.
- Having a fecal occult blood test may lead to additional testing. When the fecal occult blood test result is positive but a follow-up colonoscopy is normal, your doctor may recommend further observation with another fecal occult blood test, evaluation of your upper gastrointestinal tract, a repeat colonoscopy or a combination of these.
- Fecal occult blood tests can't detect all cancers. Some cancers detected by colonoscopy may not be detected by the fecal occult blood test.
Various foods, dietary supplements and medications can affect the results of some fecal occult blood tests — either indicating that blood is present when it isn't (false-positive) or missing the presence of blood that's actually there (false-negative). Your doctor may ask you to avoid certain foods or medicines. To ensure accurate test results, follow your doctor's instructions carefully.
For about three days before the test, your doctor may ask you to avoid:
- Certain fruits and vegetables, including broccoli and turnips
- Red meat
- Vitamin C supplements
- Pain relievers, such as aspirin and ibuprofen (Advil, Motrin IB, others)
There are several types of fecal occult blood tests, each with a different approach to collecting and testing stool. They include:
Guaiac fecal occult blood test (gFOBT). Your doctor typically gives you a test card with room for two or three samples or two or three test cards.
You collect a stool sample from each of two or three bowel movements in a clean container, usually taken on consecutive days, and then use an applicator stick to apply a smear of stool to a specific area of a card.
After the samples are dry, you return them to your doctor or a designated lab, by mail or in person.
Flushable reagent pad or tissue. You can get this kit at a store without a prescription.
You place the pad or tissue in the toilet bowl after a bowel movement, usually on three consecutive days. The pads change color when blood is present.
You then report the changes to your doctor, usually on a mail-in form.
Immunochemical fecal occult blood test (iFOBT, or FIT). The collection method for this test may depend on the manufacturer, but typically, you use a special spoon or other device to collect a sample of stool and store it in a collection container that comes with the test kit.
The collection container is then returned to your doctor or a designated lab, by mail or in person.
Immunochemical testing is newer than gFOBT. It doesn't require any dietary restrictions before sample collection, and testing can often be performed on a random stool sample. Immunochemical testing is also more sensitive than is gFOBT.
For accurate results, follow the instructions and return the samples promptly.
Your doctor will review the results of the fecal occult blood test and then share the results with you.
- Negative result. A fecal occult blood test is considered negative if no blood is detected in your stool samples. If you had the test to screen for colon cancer and you're at average risk — you have no colon cancer risk factors other than age — your doctor may recommend waiting one year and then repeating the test.
- Positive result. A fecal occult blood test is considered positive if blood is detected in your stool samples. You may need additional testing — such as a colonoscopy — to locate the source of the bleeding.