The fecal occult blood test — also known as 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. The fecal occult blood test doesn't indicate potential sources of bleeding.
Why it's done
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 gastrointestinal bleeding — such as a bleeding ulcer — is contributing to anemia.
How you prepare
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). 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, ibuprofen (Advil, Motrin, others)
What you can expect
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 (FOBT 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 gFOBT.
For accurate results, it's important to 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.
It's important to remember that false-negative and false-positive results are possible. Some cancers and most polyps don't bleed, which may lead to a false-negative result. Likewise, occult bleeding may come from sources other than colon cancer or a polyp — a false-positive — such as from a stomach ulcer, hemorrhoid, or even blood swallowed from your mouth or nose.