Fecal occult blood tests (FOBT) use chemical indicators on stool samples to detect the presence of blood not otherwise visible. Blood originating from or passing through the gastrointestinal tract can signal many conditions requiring further tests and, possibly, medical intervention. These include, but are not limited to: Colorectal and gastric cancers; ulcers; hemorrhoids; polyps; inflammatory bowel disease; and irritations or lesions of the GI tract caused by medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or aspirin, or by stomach acid disorders, such as reflux esophagitis. FOBT are used routinely (in conjunction with a rectal examination performed by a physician) to screen for colorectal cancer, particularly after age 50; the ordering of this test should not be taken as an indication that cancer is suspected. Certain foods and medicines can influence the test results. For 48 hours prior to collecting samples, avoid red meats, NSAIDs (including aspirin), antacids, steroids, iron supplements, and vitamin C, including citrus fruits and other foods containing large amounts of vitamin C.

In most cases, stool samples can be collected at home, using a kit supplied by the physician. Another name for this procedure is the hemoccult test. The standard kit contains a specially prepared card on which a small sample of stool will be spread, using a stick provided in the kit. The sample is placed in a special envelope and either mailed or brought in for analysis. When hydrogen peroxide is applied to the back of the sample, the paper will turn blue if an abnormal amount of blood is present. Many factors can result in false-positive and false-negative findings, and it is important to note that a true-positive finding only signifies the presence of blood; it is not an indication of cancer. The National Cancer Institute has found that less than 10% of all positive results were caused by cancer. Alternatively, a negative result (meaning no blood was detected) does not guarantee the absence of colon cancer, which may bleed only occasionally or not at all. The physician will want to follow up on a positive result with further tests.