This test can mainly find cancer.

This test mainly finds colorectal cancer by examining the patient’s stool. Most people find this test easier because it is not invasive and can be done at home. But it is not as good at detecting polyps, and a positive result will likely require an invasive test such as colonoscopy.

The FOBT is used to find hidden blood in feces. Blood vessels at the surface of larger colorectal polyps or cancers are often fragile and easily damaged by passing feces. The damaged vessels usually release a small amount of blood into the feces, but only rarely is there enough bleeding to be noticeable in the stool.

The FOBT detects blood in the stool through a chemical reaction. This test cannot tell whether the blood is from the colon or from other portions of the digestive tract. If this test is positive, it is not sufficient to simply repeat the FOBT or follow up with other types of tests. A colonoscopy is needed to see if there is a cancer, polyp, or other cause of bleeding such as ulcers, hemorrhoids, diverticulosis (tiny pouches that form at weak spots in the colon wall), or inflammatory bowel disease.

This screening test is done with a take-home kit that you can use in the privacy of your own home. An FOBT done during a digital rectal exam in the doctor's office is not sufficient for screening. In order to be beneficial the test must be repeated every year.

People having this test receive a kit from their doctor's office or clinic. Instructions will explain how to take a stool sample at home (usually specimens from three consecutive bowel movements that are smeared onto small squares of paper). The kit should then be returned to the doctor's office or medical lab (usually within two weeks) for testing.

Before the test: Some foods or drugs can affect the test, so your doctor may suggest that you try to avoid the following before this test:

• Non-steroidal anti-inflammatory drugs, such as ibuprofen (Advil), naproxen (Aleve), or more than one adult aspirin per day, for seven days before testing.
  
• Vitamin C in excess of 250 mg daily from either supplements or citrus fruits and juices for three days before testing.
  
• Red meats (beef, lamb, or liver) for three days before testing (components of blood in the meat may cause the test to show positive.)

Some people who are given the kit never do the test or return samples to their doctor because they worry that something they ate may interfere with the test. For this reason, many doctors tell their patients it isn't essential to follow any restrictions in their diet. The most important thing is to get the test done.