The test described above is called a fecal occult blood test (FOBT), also referred to as a hemoccult test or Guaiac card. A patient is given three cards to take home and asked to put a small sample of stool on a card on three separate occasions. Technically, the FOBT is a screening test for cancer that looks for microscopic amounts of blood in the stool. The concept behind this test is that large polyps (benign growths that have the potential to turn cancerous) and cancers may pull away from the intestinal wall that drip small amounts of blood into the stool that may be invisible to the naked eye.
Unfortunately, these tests are very inaccurate. In fact, some physicians choose not to do them at all. Even when positive, there is only about a 2% chance that the bleeding is caused by cancer. And there are some foods that can produce a false positive result. However, the FOBT is inexpensive, simple to use, and easily administered. The FOBT is rarely used alone. Other colon cancer screening modalities such as a flexible sigmoidoscopy or colonoscopy are used to visualize the colon directly. However, I have found that the FOBT allows patients to become involved and educated about colorectal cancer screening and prevention. It should be emphasized that, like rectal bleeding, the chances are good that a positive test will turn out to be caused by factors other than cancer.
