If blood is present, the paper turns blue. Because the reaction depends on the presence of iron in the blood, patients should not take iron supplements or eat red meats several days before the test. 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.
Any abnormal result requires further tests.
Some experts also argue that the 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 Other Techniques Used to Visualize the Colon.