Fecal Occult Blood Test
The fecal occult blood test (FOBT) is performed as part of the routine physical examination
during the examination of the rectum. It is used to detect microscopic blood in the stool and is a screening tool for colorectal cancer
FOBT uses chemical indicators on stool samples to detect the presence of blood not otherwise visible. (The word "occult" in the test's name means that the blood is hidden from view.) Blood originating from or passing through the gastrointestinal tract can signal many conditions requiring further diagnostic procedures and, possibly, medical treatment. These conditions may be benign or malignant and some of them include:
- colon cancer, rectal cancer, and gastric cancers
- inflammatory bowel disease
- irritations or lesions of the gastrointestinal tract caused by medications (such as nonsteroidal anti-inflammatory drugs, also called NSAIDs)
- irritations or lesions of the gastrointestinal tract caused by stomach acid disorders, such as reflux esophagitis
The FOBT is 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. The FOBT must be combined with regular screening endoscopy (such as a sigmoidoscopy
) to detect cancers at an early stage.
Certain foods and medicines can influence the test results. Some fruits contain chemicals that prevent the guaiac, the chemical in which the test paper is soaked, from reacting with the blood. Aspirin
and some NSAIDs irritate the stomach, resulting in bleeding, and should be avoided prior to the examination. Red meat and many vegetables and fruits containing vitamin C also should be avoided for a specified period of time before the test. All of these factors could result in a false-positive result.
Feces for the stool samples is obtained either by the physician at the rectal examination
or by the patient at home, using a small spatula or a collection device. In most cases, the collection of stool samples can easily be done at home, using a kit supplied by the physician. 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 the physician applies hydrogen peroxide to the back of the sample, the paper will turn blue if an abnormal amount of blood is present.
Types of fecal occult blood tests
Hemoccult is the most commonly used fecal occult blood test. The Hemoccult test takes less than five minutes to perform and may be performed in the physician's office or in the laboratory. The Hemoccult blood test can detect bleeding from the colon as low as 0.5 mg per day.
Tests that use anti-hemoglobin antibodies (or immunochemical tests) to detect blood in the stool are also used. Immunochemical tests can detect up to 0.7 mg of hemoglobin in the stool and do not require dietary restrictions. Immunochemical tests
- are not accurate for screening for stomach cancer
- are more sensitive than Hemoccult tests in detecting colorectal cancer
- are more expensive than Hemoccult tests.
Hemoquant, another fecal occult blood test, is used to detect as much as 500 mg/g of blood in the stool. Like the Hemoccult, the Hemoquant test is affected by red meat. It is not affected by chemicals in vegetables.
Fecal blood may also be measured by the amount of chromium in the red blood cells in the feces. The stool is collected for three to ten days. The test is used in cases where the exact amount of blood loss required. It is the only test that can exclude blood loss from the gastrointestinal area with accuracy.
Medicare coverage began on January 1, 2004, for a newer fecal occult blood test based on immunoassay. This technique does not rely on guiaic, so it is not influenced by diet or medications used prior to the test. The immunoassay test also requires fewer specimen collections. At a conference of gastroenterologists (physicians who specialize in diseases of the stomach and related digestive systems), a company announced a new fecal occult blood test that was based on DNA and appeared more sensitive than traditional tests. Widespread use of these new tests remains to be seen; the traditional guiaic test has been in place for about 30 years.
For 72 hours prior to collecting samples, patients should 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. Foods like uncooked broccoli, uncooked turnips, cauliflower, uncooked cantaloupe, uncooked radish and horseradish and parsnips should be avoided and not eaten during the 72 hours prior to the examination. Fish, chicken, pork, fruits (other than melons) and many cooked vegetables are permitted in the diet.
Many factors can result in false-positive and falsenegative findings.
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 states that, in its experience, less than 10% of all positive results were caused by cancer. The FOBT is positive in 1-5% of the unscreened population and 2-10% of those are found to have cancer. The physician will want to follow up on a positive result with further tests, as indicated by other factors in the patient's history or condition.
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. (Only 50% of colon cancers are FOBT-positive.)
Screening using the FOBT has been demonstrated to reduce colorectal cancer. However, because only half of colorectal cancers are FOBT-positive, FOBT must be combined with regular screening endoscopy to increase the detection of pre-malignant colorectal polyps and cancers. Since, through FOBT, cancer may be detected early, the benefits of possible early detection must be considered along with the likelihood of complications and costs for additional studies.
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— Not visible or easily detected.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.