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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 now 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. Current guidelines recommend screening with a guaiac-based test such as Hemoccult II, which has been classified as a waived test by CLIA '88. Guaiac based FOBT make use of the pseudoperoxidase activity of hemoglobin. Guaiac turns blue after oxidation by oxidants or peroxidases in the presence of an oxygen donor such as hydrogen peroxide. The likelihood that a guaiac-based test will be positive is proportional to the quantity of fecal heme, which in turn is related to the size and location of the bleeding lesion. FOBT are optimally designed to detect large, distal lesions. Generally, 10 mL of daily blood loss is required for Hemocult II tests to be positive 50% of the time. Patients should collect a total of six samples in order to compensate for sampling error since blood is not evenly distributed in stool. Two slides should be prepared from each of three consecutive bowel movements. The collection should be made 24 to 48 hours after eating a meat free diet and avoidance of vitamin C, aspirin, and nonsteroidal anti-inflammatory drugs. Slides need to be developed within 7 days of collection. Longer periods of storage cause weakly positive stools to become falsely negative. The 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 up to 16%. A false positive rate of this magnitude leads to too many nonproductive colonoscopic examinations and makes screening impractical. FOBT is considered positive if even one of the six slide windows turns blue. A middle-aged adult with a positive result on an initial FOBT (performed without slide rehydration) has a 7 to 14% probability of early colorectal cancer (Dukes stage A or B). The probability of early cancer or a large (>1 cm) adenoma is approximately 30%. Cancer detection rates are lower following rescreening. Nonetheless, cancer detection rates are high enough to warrant a complete evaluation of the colon and rectum whenever a person has a positive test either initially or at rescreening. If the results of colonoscopy are negative, FOBT does not need to be repeated for 5 years. If colonoscopy reveals cancer or a high-risk adenoma, periodic colonoscopic surveillance is indicated. A negative result on FOBT does not rule out colorectal cancer, because the sensitivity of the test is only 30 to 50%. 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. 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. |
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