Screening for colorectal cancer can lower the risk for dying of colorectal cancer by finding growths (adenomas) before they become cancer or by finding cancer at an early, curable stage. One commonly used screening test for colorectal lesions is the fecal occult blood test (FOBT). The traditional guaiac-based FOBT uses a chemical reaction on a paper card to find traces of blood that leak from adenomas and cancer into the stool. The guaiac-based FOBT result can be positive when there is no source of bleeding (false-positive) if, for example, a person has recently eaten undercooked meats. The guaiac-based FOBT result may also be negative when there is a source of bleeding (false-negative) if a person has been taking vitamin C supplements.