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Fecal Occult Blood Test (FOBT)

Colorectal cancer is the only major cancer that affects men and women almost equally.  It is rare in persons under age 40, but the incidence begins to rise substantially after age 50. About 6% of people develop colorectal cancer by 80 years of age and 50% die as a result of the cancer. Three recent randomized, controlled trials have convincingly shown that the mortality rate can be reduced 15 to 35% by screening with fecal occult blood tests (FOBT).   As a result of these studies, major professional organizations such as the American Cancer Society, the United States Preventative Service Task Force, the American College of Physicians, and the College of American Pathologists recommend annual testing of all adults at 50 years of age or older.  FOBT was also added to the list of approved Medicare Preventive Service Benefits on January 1, 1998.  FOBT is based upon the principle that advanced adenomas and colorectal cancer are friable lesions that leak trace amounts of blood as a result of stool trauma and contractions of the colon.


Guaiac testing FOBT has been classified as a waived test by CLIA ‘88. A number of waived kits are available commercially. These kits contain cards on which a small amount of feces is smeared in a designated space, the flap is closed, and a hydrogen peroxide developer is added to the opposite side of the window. If blood is present in sufficient quantity a blue color appears on the card.


When blood leaks into the gastrointestinal (GI) tract, red blood cells are lysed, releasing hemoglobin, which is subsequently degraded to hematin and globin. Hematin is then metabolized by colonic bacteria to porphyrin. Guaiac based FOBT make use of the pseudoperoxidase activity of hemoglobin and hematin. When guaiac is mixed with hydrogen peroxide and fecal material containing blood, the peroxidase activity of hemoglobin and hematin oxidizes guaiac, turning it from a neutral to a blue color. Generally, guaiac-based FOBT can detect as little as 10 mL per day of blood loss, which is about 5-10 times more than the normal daily blood loss.


Patients should be instructed to avoid meat and certain vegetables24 to 48 hours prior to collecting stool samples for FOBT. Red meat contains bovine hemoglobin which can produce a positive reaction. Foods containing peroxidase such as radishes, turnips, cabbage, cauliflower, horseradish, uncooked broccoli, cucumbers, mushrooms, green beans, artichokes and melons can generate falsely positive color reactions. Some kit manufacturers include alcohol in the developer to denature plant peroxidases. Fruit juices, which contain Vitamin C (ascorbic acid), and vitamin C supplements, should be avoided because they may inhibit the guaiac oxidation reaction, producing a falsely negative result.


The overall specificity of guaiac FOBT has varied in the literature, ranging from 88% to 98%. Compared to colonoscopy, sensitivity is only 12% for advanced adenomas and 26% for colorectal cancer.


For every 100 asymptomatic patients aged 45 years or older, about five will have a positive stool guaiac test if they are not on a rigid peroxidase-free diet, and about 2 will have a positive test if they are. Approximately 20 to 30% of the latter group will have a benign tumor such as a polyp and about 10-15% will have colorectal carcinoma. Most of these detected malignancies are in early stages of growth and have 5 year survival rates of at least 80%.


Approximately 30 to 60% of guaiac positive subjects will harbor gastrointestinal lesions other than cancer or polyps that might be responsible for occult bleeding. These lesions include hemorrhoids, diverticulosis, inflammatory bowel disease, peptic ulcer, gastritis, esophagitis, and esophageal varices.  In approximately 15% of subjects with positive FOBT, the source of bleeding cannot be determined. Reasons include ingestion of vegetable peroxidase or the presence of a lesion too small to be detected.


Bleeding from small cancers and polyps tends to be intermittent, so a negative FOBT does not rule out its presence. To increase the likelihood of detecting blood, patients should be instructed to test three separate stool samples on three different days. With some tests, the clinical sensitivity of FOBT increases from 30% for a single test to 90% for three tests.


A negative result on FOBT does not rule out colorectal cancer, because the sensitivity of the test is so low.  FOBT should be repeated either annually or biennially.  If symptoms develop that suggest colorectal cancer a more definitive test should be performed to rule out a neoplasm.


Medications such as aspirin and nonsteroidal anti-inflammatory agents (NSAIDs) may cause gastrointestinal bleeding and produce a positive FOBT. For this reason, patients should be instructed to stop taking these medications at least three days prior to testing. Other medications that might cause bleeding include corticosteroids and anticoagulants.


Antacids and anti-diarrheal medications containing bismuth render the stool dark and may confound the reading of FOBT. Oral iron supplements give the stool a dark-green or black appearance that may be confused with the blue color of a positive guaiac test.


Guaiac tests perform well in detecting GI bleeding that originates in the colon, but are not as effective in detecting lesions in the stomach and small intestine. Intestinal bacteria are so efficient in breaking down hemoglobin it is usually not detectable by the time it reaches the colon. Occasionally, an upper gastrointestinal bleed may be so large that not all the hemoglobin is degraded and the guaiac test will be positive. 


FOBT should not be performed on stool samples taken by digital rectal examination because the exam, itself, may cause minor bleeding. If the test is positive, it will be impossible to determine the source of the bleeding.


Slides need to be developed within 7 days of collection. Longer periods of storage cause weakly positive stools to become falsely negative.  Dried stool specimens should not be rehydrated with a drop of water at the time of development because this practice increases the false positive rate by 16%.  A false positive rate of this magnitude leads to too many nonproductive colonoscopic examinations and makes screening impractical. 


To improve from the sensitivity of the most commonly used guaiac FOBT, Hemoccult, Beckman Coulter developed two newer versions, Hemoccult II and the Hemoccult II SENSA. Hemoccult II has a sensitivity of 25% to 38% and a specificity of 98% to 99% according to the US Preventive Services Task Force. Hemoccult SENSA has a sensitivity of 62% to 79% and a specificity of 87% to 96%. Although this test has improved sensitivity over the previous versions, it has lower specificity and, therefore, more false-positive results.

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