Some foods can cause the results of gFOBT to be a “false positive,” which means it appears that there is blood in the patient's stool even there is no blood in the stool. You need to avoid these foods 2 to 3 days before the test: turnips, beets, radishes, horseradish, artichokes, mushrooms, broccoli, bean sprouts, cauliflower, apples, oranges, bananas, grapes and melon.