Polyps are noncancerous outgrowths of the wall of the colon or rectum that can become cancerous.
Screening tests prevent some colorectal cancers by detecting and removing polyps before they become cancers and by detecting cancers in their early stages, when they can still be cured with surgery. Fecal occult blood testing uses a simple chemical reaction to detect blood hidden in stool. Since polyps and colorectal cancers can cause blood to leak into the stool, a positive FOBT suggests the need for further testing. Fecal occult blood testing is inexpensive, colonoscopy is very expensive, and costs of sigmoidoscopy are somewhere in between. It is not clear which screening strategy makes the most sense in terms of costs and associated benefits.