Because the vast majority of positive FOBT results are false-positives, the likelihood that a positive test indicates cancer is relatively low. Over the past several years, there has been a shift of tumors from the rectum and descending colon toward more proximal lesions in the ascending colon. Screening for colorectal cancer is recommended for all persons 50 years and older with annual FOBT or sigmoidoscopy, or both. There is insufficient evidence to determine which of these screening methods is preferable or whether the combination of FOBT and sigmoidoscopy produces greater benefits than either test alone. Although there is good evidence to support use of FOBT on an annual basis, there is insufficient evidence to recommend a periodicity for sigmoidoscopy screening.
Scientific evidence supporting barium enema or colonoscopy as routine screening is more limited.
If a single test panel for a FOBT is positive, follow-up requires diagnostic procedures such as sigmoidoscopy, colonoscopy or barium enema. Likewise, positive results from flexible sigmoidoscopy also require colonoscopy. Despite previous reports, dietary iron does not cause false-positive test results. Screening for colorectal cancer is recommended for all persons 50 years and older. In terms of quantifying risk, it is highest among persons with genetic syndromes such as hereditary polyposis and hereditary non-polyposis colorectal cancers, as well as among persons who have had longstanding ulcerative colitis.