Screening tests for CRC include stool-based tests and visual exams. Stool-based tests rely on the fact that polyps and cancers contain blood vessels that damage easily and bleed into the colon or rectum. The older guaiac-based fecal occult blood test can detect blood in the stool with a chemical reaction. This test has limited sensitivity, and is not specific for human blood, so it requires dietary restrictions prior to its use. Fecal immunochemical tests rely on immunochemical processes to detect blood, and do not require pretest dietary restrictions. New stool DNA tests, detect blood or DNA mutations associated with CRC in the stool. A positive result to any of these stool tests should be followed up with colonoscopy. These tests may also be associated with false-positive results, requiring a follow-up colonoscopy.
Visual exams to screen for CRC include colonoscopy, in which a physician uses a colonoscope to visualize the entire length of the rectum and colon. The colonoscope can accommodate specialized instruments designed to remove polyps or obtain a biopsy if suspicious areas are found. A virtual colonoscopy can also be performed using computed tomography (CT) technology, which involves the use of x-rays and a computer. A procedure similar to colonoscopy can be performed with a flexible sigmoidoscope. However, because the sigmoidoscope is much shorter than a colonoscope, this procedure can only visualize the rectum and sigmoid colon. It is no longer widely used in the United States. Guidelines issued by the American Cancer Society recommend that patients at average risk over age 45 be screened with a stool-based test or visual exam. Tap the buttons for Average Risk and High Risk to explore screening recommendations for patients with different levels of CRC risk.