Colon Cancer Screening
Colon cancer is the third leading cause of cancer in men and women in the United States and the second leading cause of cancer-related death; however, the death rate for colon cancer has decreased by approximately 15% over the past 10 years because of more frequent and more effective screening. Screening recommendations include annual fecal occult blood testing (FOBT), flex sigmoidoscopy every 5 years, and full colonoscopy every 10 years after the age of 50.[65,66] Since patients are often reluctant to submit to sigmoidoscopy and colonoscopy, FOBT has become the standard for screening in average-risk individuals.[65,66,67] Multiple clinical trials have shown that annual FOBT can decrease morbidity and mortality associated with colorectal cancer.[65,67,68,69] Simple home and outpatient screening tests have become a mainstay for the FOBT market.
Two types of FOBT products are currently available for the home and ambulatory care settings ( Table 4 ).[65,70,71,72,73,74,75] The most commonly used device is the guaiac test, which works via the oxidation of guaiac in the test strip by hydrogen peroxide in the reagent. The iron in the heme portion of the hemoglobin molecule catalyzes the reaction, and the reaction will not proceed without hemoglobin.[72,73,74] This oxidative process causes a change in color at the reaction site. The second type of FOBT is fecal immunochemical testing (FIT), which relies on antibodies to hemoglobin forming immune complexes.[70,71]
Guaiac test products are significantly less expensive than FIT products, and the test may be conducted at home. FIT products are intended for analysis in a health care setting; however, the testing procedure has a Clinical Laboratory Improvement Amendments waiver and may be a useful option for many clinicians. Patients may obtain samples at home and return to the clinic or laboratory for analysis and interpretation of results. Guaiac test products have much lower average sensitivity (50%) than FIT devices (94%).[70,71,72,73,74,75] All guaiac test products require at least three tests of consecutive stools to determine a positive or negative result, with many clinicians suggesting at least four to six separate tests.[72,73,74] FIT devices require only two consecutive tests to reach a conclusive result and seldom yield contradictory results.[70,71,75]
Many older guaiac test products (including Hemoccult, Beckman Coulter) require patients to avoid red meat and foods that contain peroxidase for at least three days prior to testing (the Hemoccult test does not differentiate between human hemoglobin and animal hemoglobin). Raw foods high in peroxidase (e.g., horseradish, pineapple, red fruits) may serve to oxidize the guaiac despite an absence of dietary hemoglobin, again resulting in a false-positive result. Each of these restrictions is avoided with FIT products, since the antibody responsible for the reaction is specific to human hemoglobin. The newer flushable-reagent guaiac test products do not impose dietary restrictions,[72,73] nor do they require much manipulation of the stool sample, which may promote patient compliance.[76,77]
Dietary restrictions must be strictly adhered to with the older guaiac test devices to avoid false-positive results. In addition to observing limitations in diet, the patient must avoid aspirin in dosages of greater than 325 mg/day, non-steroidal antiinflammatory drugs, corticosteroids, phenylbutazone, reserpine, anticoagulants, antimetabolites, and antineoplastic agents for at least seven days prior to using all guaiac test devices. (If a patient is taking any of these medications, the prescribing physician should be consulted prior to their discontinuation.) Supplementary vitamin C in dosages of greater than 250 mg/day may result in false negatives.[72,73,74]
Both types of FOBT devices respond to blood in the feces, so a positive result is not specific for colon cancer. These tests are not useful in patients who have hemorrhoids or other known causes of rectal bleeding. Many colorectal cancers may bleed intermittently, so taking at least three samples after three separate bowel movements is important to optimize the likelihood of detecting a cancer. Because of the lack of quantitative results with FOBT, all guaiac test findings should be discussed with a health care provider. Since FIT is performed in a clinic setting, results will be reviewed by the provider before being given to the patient. Results are often inconclusive and ambiguous, necessitating further testing (often sigmoidoscopy or colonoscopy). Patients should be aware that these tests are not diagnostic and are simply screens to determine if further testing is required.
Am J Health Syst Pharm. 2008;65(4):299-314. © 2008 American Society of Health-System Pharmacists
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