Bleeding in the digestive tract can lead to blood in the stool. In fact, having small traces of blood in the stool is normal. Fecal blood in amounts greater than normal, but not noticeable visually, is called occult blood. The test detects digestive tract bleeding in the stool and is used to screen for colon and rectal cancer. Colon cancer is a major cause of death in the United States. Regular testing can therefore be a life-saving routine. Healthcare providers recommend regular, occult blood tests starting at age 50.

Before the test, the patient should eat a high fiber diet for 2 or 3 days. He or she should avoid certain raw foods, turnips,horseradish, and also red meat. Testing may take place in a lab or clinic. Healthcare providers may also give the person a kit to collect samples at home.

Three or more stool specimens are collected. The test involves mixing a small sample of stool with a chemical that reacts with blood. If there are normal amounts of blood, the reaction is green. If there is occult blood, the test turns dark blue within 5 minutes. A common home kit contains a folder that has 3 panels containing the chemical. It also provides 3 flat sticks for applying the samples to the panels. The person seals the folder and mails it to a medical lab. Lab technicians process the panels and observe the reaction.

A positive test result may indicate polyps in the colon or rectum. Polyps are not cancer, but they can become cancer. Other reasons for digestive tract bleeding include hemorrhoids, burst blood vessels, ulcers, ulcerative colitis, dysentery, or simply diet. Further tests are necessary to define the cause. These may include:
* endoscopy, when a flexible telescope is used to examine the intestines, or
* a barium swallow, or a barium enema, where X-rays are taken after contrast material is swallowed by mouth or given by enema.

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