Several screening tests have been developed to help doctors find colorectal cancer early, as well as adenomas and other polyps. Like many other cancers, colorectal cancer is usually more treatable when found before it causes symptoms or has a chance to spread. And because these tests allow growths that might otherwise become cancer to be detected and removed, colorectal cancer screening may be a form of cancer prevention, not just early detection.
What methods are used to screen people for colorectal cancer?
Expert medical groups, including the United States Preventive Services Task Force, strongly recommend screening for colorectal cancer. Although minor details of the recommendations may vary, these groups generally recommend that people at average risk of colorectal cancer get screened at regular intervals with high-sensitivity fecal occult blood tests (FOBT), sigmoidoscopy, or colonoscopy beginning at age 50 (2).
People at increased risk because of a family history of colorectal cancer or polyps or because they have inflammatory bowel disease or certain inherited conditions may be advised to start screening before age 50 and/or have more frequent screening.
High-sensitivity fecal occult blood tests (FOBT)
: Both polyps and colorectal cancers can bleed, and FOBT checks for tiny amounts of blood in feces (stool) that cannot be seen. (Blood in stool may also indicate the presence of benign conditions, such as hemorrhoids.)
Currently, two types of FOBT are approved by the Food and Drug Administration to screen for colorectal cancer: guaiac FOBT (gFOBT) and the fecal immunochemical (or immunohistochemical) test (FIT, also known as iFOBT). With both types of FOBT, stool samples are collected by the patient using a kit, and the samples are returned to the doctor.
- Guaiac FOBT uses the chemical guaiac to detect heme, the iron-containing component of the blood protein hemoglobin. Because the guaiac FOBT can also detect hemoglobin from dietary sources, people have to observe certain dietary restrictions before having this test.
- FIT uses antibodies to detect human hemoglobin protein (3, 4). Dietary restrictions are typically not required for FIT.
Studies have shown that guaiac FOBT, when performed every 1 to 2 years in people aged 50 to 80 years, can help reduce the number of deaths due to colorectal cancer by 15 to 33 percent (3, 4). If FOBT is the only type of colorectal cancer screening test performed, expert groups recommend yearly testing (2).
: In this test, the rectum and sigmoid colon are examined using a flexible lighted instrument called a sigmoidoscope, which is inserted through the anus into the rectum and sigmoid colon as air (or carbon dioxide) is pumped into the colon to expand it so the doctor can see the colon lining more clearly.
During sigmoidoscopy, abnormal growths in the rectum and sigmoid colon can be removed for analysis (biopsied). The lower colon must be cleared of stool before sigmoidoscopy, but the preparation is less involved than that required for colonoscopy. People are usually not sedated for this test.
Studies have shown that people who have regular screening with sigmoidoscopy after age 50 years have a 60 to 70 percent lower risk of death due to cancer of the rectum and lower colon than people who do not have screening (5, 6).
One randomized controlled clinical trial found that even just one sigmoidoscopy screening examination between 55 and 64 years of age can substantially reduce colorectal cancer incidence and mortality (7). Expert groups generally recommend sigmoidoscopy every 5 years along with FOBT every 3 years for people at average risk who have had negative test results (2).
Standard (or optical) colonoscopy
: In this test, the rectum and entire colon are examined using a flexible lighted instrument called a colonoscope, which is inserted through the anus into the rectum and the colon as air (or carbon dioxide) is pumped into the colon to expand it so the doctor can see the colon lining more clearly.
During colonoscopy, any abnormal growths in the colon and the rectum can be removed, including growths in the upper parts of the colon that are not reached by sigmoidoscopy. A thorough cleansing of the colon is necessary before this test. Most patients receive some form of sedation during the test.
Studies suggest that colonoscopy reduces deaths from colorectal cancer by about 60 to 70 percent, although randomized controlled clinical trials that will provide more definitive information about the size of the mortality reduction are under way (8). Expert groups recommend colonoscopy every 10 years for people at average risk as long as their test results are negative (2).
: Although most expert groups generally recommend high-sensitivity FOBT, sigmoidoscopy, and colonoscopy as standard colorectal cancer screening tests, several other types of tests exist.
One is virtual colonoscopy (also called computed tomographic [CT] colonography), a fairly new method of screening in which special x-ray equipment (a CT scanner) is used to produce pictures of the colon and the rectum from outside the body. A computer then assembles these pictures into detailed images that can show polyps and other abnormalities. Virtual colonoscopy is less invasive than standard colonoscopy and does not require sedation.
As with standard colonoscopy, a thorough cleansing of the colon is necessary before this test, and air (or carbon dioxide) is pumped into the colon to expand it for better viewing of the colon’s lining. The accuracy of virtual colonoscopy is similar to that of standard colonoscopy and it has a lower risk of complications. However, if polyps or other abnormal growths are found during a virtual colonoscopy, a standard colonoscopy is usually performed to remove them.
Whether virtual colonoscopy can help reduce deaths from colorectal cancer is not yet known, and Medicare and most insurance companies do not currently reimburse the costs for this procedure. Studies are ongoing to compare virtual colonoscopy with other screening methods.
Double-contrast barium enema (DCBE) is another method of visualizing the colon from outside the body. In DCBE, a series of x-ray images of the entire colon and rectum is taken after the patient is given an enema with a barium solution. The barium helps to outline the colon and the rectum on the images. DCBE is not widely used because it is less sensitive than colonoscopy for detecting small polyps and cancers. However, it may be used for people who cannot undergo standard colonoscopy—for example, because they are at particular risk for complications.
Finally, doctors sometimes perform a single-specimen guaiac FOBT on a stool sample collected during a digital rectal examination done as part of a routine physical exam. However, this approach has not been shown to be an effective way to screen for colorectal cancer (9).