# What methods are used to screen people for colorectal cancer?
Health care providers may suggest one or more of the following tests for colorectal cancer screening:
* Fecal occult blood test (FOBT)— This test checks for hidden blood in fecal material (stool). Currently, two types of FOBT are available. One type, called guaiac FOBT, uses the chemical guaiac to detect heme in stool. Heme is the iron-containing component of the blood protein hemoglobin. The other type of FOBT, called immunochemical FOBT, uses antibodies to detect human hemoglobin protein in stool (13–15). Studies have shown that FOBT, when performed every 1 to 2 years in people ages 50 to 80, can help reduce the number of deaths due to colorectal cancer by 15 to 33 percent (13–15).
* Sigmoidoscopy— In this test, the rectum and lower colon are examined using a lighted instrument called a sigmoidoscope. During sigmoidoscopy, precancerous and cancerous growths in the rectum and lower colon can be found and either removed or biopsied. Studies suggest that regular screening with sigmoidoscopy after age 50 can help reduce the number of deaths from colorectal cancer (14). A thorough cleansing of the lower colon is necessary for this test.
* Colonoscopy—In this test, the rectum and entire colon are examined using a lighted instrument called a colonoscope. During colonoscopy, precancerous and cancerous growths throughout the colon can be found and either removed or biopsied, including growths in the upper part of the colon, where they would be missed by sigmoidoscopy. However, it is not yet known for certain whether colonoscopy can help reduce the number of deaths from colorectal cancer. A thorough cleansing of the colon is necessary before this test, and most patients receive some form of sedation.
* Virtual colonoscopy (also called computerized tomographic colonography)—In this test, special x-ray equipment is used to produce pictures of the colon and rectum. A computer then assembles these pictures into detailed images that can show polyps and other abnormalities. Because it is less invasive than standard colonoscopy and sedation is not needed, virtual colonoscopy may cause less discomfort and take less time to perform. As with standard colonoscopy, a thorough cleansing of the colon is necessary before this test. Whether virtual colonoscopy can reduce the number of deaths from colorectal cancer is not yet known.
* Double contrast barium enema (DCBE)—In this test, a series of x-rays of the entire colon and rectum are taken after the patient is given an enema with a barium solution and air is introduced into the colon. The barium and air help to outline the colon and rectum on the x-rays. Research shows that DCBE may miss small polyps. It detects about 30 to 50 percent of the cancers that can be found with standard colonoscopy (14).
* Digital rectal exam (DRE)—In this test, a health care provider inserts a lubricated, gloved finger into the rectum to feel for abnormal areas. DRE allows examination of only the lower part of the rectum. It is often performed as part of a routine physical examination.
Scientists are still studying colorectal cancer screening methods, both alone and in combination, to determine how effective they are. Studies are also under way to clarify the potential risks, or harms, of each test. See Question 5 for a table outlining some of the advantages and disadvantages, including potential harms, of specific colorectal cancer screening tests.