Most cases of colon cancer begin with the development of benign intestinal polyps. Benign polyps are relatively common in people over the age of 50, and while most do not cause health problems, some can become cancerous and potentially spread to other parts of the body (metastasize). These finger-like growths protrude into the intestinal cavity (lumen) or the rectum. They can be fragile and bleed intermittently, such as when food waste brushes against them.

The blood released is not usually visible in the stool, but it can be detected with a fecal occult blood test (FOBT) or fecal immunochemical test (FIT). This small amount of blood may be the first and sometimes the only sign of early colon cancer, making the FOBT and FIT valuable screening tools for colorectal (colon and rectal) cancer. Methods for testing include a guaiac-based test (gFOBT), an over-the-counter (OTC) flushable reagent pad, and an immuochemical method (iFOBT or FIT).

It is recommended that testing be performed on at least three stool samples collected on different days. The American Cancer Society has stated that a single test performed at the time of a digital rectal exam in a health practitioner's office is not recommended because it may not be sensitive enough to screen for cancer. The home FOBT or FIT is recommended because the collection of stool on three different days increases the chance of detecting cancer. Furthermore, it is recommended that those who choose this method of colon cancer screening get screened every year.

Each method has a different approach to collecting and testing stool samples for occult blood.

• For the guaiac-based test (gFOBT), the health practitioner or laboratory will typically provide three test cards. Separate stool samples are collected from different bowel movements, usually on three consecutive days. For each test, a stool sample should be collected into a clean container and should not be contaminated with urine or water. A test card is labeled with the person's name and the date; then, with an applicator stick, a thin smear of stool is put onto a designated area on the card and allowed to dry. Once it is dry, it is stable for several weeks at room temperature. When all of the consecutive samples have been collected and dried, the test cards are returned to the health practitioner or laboratory, usually by mailing them.
• With the OTC flushable reagent pad/tissue method, a test pad/tissue is placed in the toilet after a bowel movement. The pad contains a chemical that produces a color change when blood is present. The person doing the test watches for the characteristic color change and records the findings on a report form. Like the gFOBT, this test is usually done on three consecutive days and then the completed form is returned to the health practitioner. Be aware that use of toilet bowl cleansers or the presence of blood derived from urine or a woman's menstrual period may alter results.
• With the fecal immunochemical tests (FIT), the collection method may vary based on the specific manufacturer. A common approach is to use a special long-handled brush or other device to collect a sample from the surface of a stool sample. The brush or device is then used to transfer the sample to a special collection card. After the card is allowed to dry, it is returned to the health practitioner or laboratory. At least three such samples are collected on different days and sent in one mailing.

For guaiac-based FOBT and OTC flushable methods, there are special dental, dietary, and drug restrictions. These tests detect any blood that enters the digestive tract. Therefore, steps that are taken to avoid introducing blood into the digestive tract will increase the quality of the test sample.

• Blood that arises from bleeding gums (following dental procedures or gum disease) may be detected by these tests. Avoid having any dental procedures up to three days before beginning to collect stool samples.
• Bleeding in the stomach may be triggered by use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, naproxen, and ibuprofen. Avoid taking these drugs for seven days prior to testing, if clinically possible.
• The guaiac-based gFOBT and the OTC flushable pad/tissue rely on a chemical reaction to produce the color change that gives a positive test. Foods such as red meat, broccoli, turnips, cauliflower, apples, oranges, mushrooms, and horseradish, and drugs such as colchicine and oxidizing drugs (like iodine and boric acid) may also trigger the same chemical reaction and make the test appear positive even in the absence of human blood (a false-positive result). If instructed to do so, avoid these foods and drugs three days prior to and during the testing period.
• Vitamin C, on the other hand, interferes with the chemical reaction and prevents the color formation that should occur when blood is present (a false-negative result). Vitamin C supplements and fruit juices that contain vitamin C should be avoided three days prior to and during testing.

Follow the instructions that are provided by your health practitioner or included in test kit instructions. Check with your health practitioner before stopping any drugs to be certain that it is safe to do so.

For fecal immunochemical test (FIT) methods, there are no dietary, drug, or dental procedure restrictions. The test uses antibodies to detect only human blood from the lower digestive tract (colon).