During this period, they should also avoid eating certain raw fruit and vegetables, including cauliflower, horseradish, radishes, melons, and turnips, which contain the chemical peroxidase and can cause a positive test reaction even if no blood is present. People should delay the test if they are experiencing bleeding from other causes, such as menstruation, hemorrhoids, gingivitis, or urinary infections. Vitamin C and foods rich in this vitamin may cause a false negative reaction and should be avoided a few days before the test. Some experts also argue that the FOBT test misses too many cancers and should not be relied on. Controversy has been on-going as to whether this test is too inaccurate to be very beneficial, both in missing cancers and in showing false positive results that lead to invasive and expensive tests, most of which turn out to be unnecessary.
Compliance is a major problem.
Occult-blood tests that give results at home are available but are extremely inaccurate. Colonoscopy, Sigmoidoscopy, and Barium Enema. If a digital rectal examination or occult blood tests show signs of trouble, several methods to visualize the colon are available, including colonoscopy, sigmoidoscopy, or a double-contrast barium enema. Sigmoidoscopy can only view the rectum and the left side of the colon, while colonoscopy and barium enemas allow a view of the entire large intestine. Both flexible sigmoidoscopy and colonoscopy involve snaking a fiberoptic tube through regions of the rectum and colon to view the walls of the intestine.