Recommended screening tests include fecal occult blood testing (FOBT), sigmoidoscopy, colonoscopy, and barium enema. Fecal occult blood testing uses a chemical reaction to find traces of blood in stool. A positive result on FOBT should lead to colonoscopy. Sigmoidoscopy involves looking into the rectum and lower colon through a flexible tube-shaped instrument. Colonoscopy uses a similar but longer instrument to look at the entire colon. Doctors can take samples of the colon (biopsies) and remove polyps during colonoscopy. Barium enema involves taking x-rays of the abdomen after inserting barium into a person's colon by enema. Barium appears white on x-ray. If x-rays indicate a lesion in the colon, follow-up testing with colonoscopy is necessary.