Several tests available for colorectal cancer screening can detect precancerous polyps (adenomas) and can lead to cancer prevention and/or detect cancers at an early, more treatable stage.

Guidelines from expert groups recommend that you and your health care provider discuss the available options and choose a testing strategy that works best for you.


Colonoscopy allows a clinician to see the lining of the entire colon, including the rectum.

  • Procedure: Colonoscopy requires that you prepare by cleaning out your entire colon so the doctor can see the inside well. This clean out usually involves drinking a laxative liquid preparation that causes temporary diarrhea. During colonoscopy, you may be given a mild sedative drug or some doctors use a stronger anesthetic agent that puts you to sleep. A thin, flexible, lighted tube is inserted through the anus and used to directly inspect the lining of the rectum and the entire colon. Biopsies (samples of tissue) may be taken during the procedure. Polyps and some cancers can be removed during this procedure.
  • Effectiveness: Colonoscopy is the most sensitive of the available tests; it detects most small polyps and almost all large polyps and cancers and substantially lower the risk of developing and dying from colorectal cancer.
  • Risks and disadvantages: The risks of colonoscopy, while small, are greater than those of other screening tests. Colonoscopy may lead to serious bleeding or a tear of the intestinal wall in some individuals (about 1 out of every 1000 people). Because the procedure usually requires sedation, you must be accompanied home after the procedure and you should not return to work or other activities on the same day.


Sigmoidoscopy allows a clinician to directly view the lining of the rectum and the lower part of the colon. This area accounts for about one-half of the total area of the rectum and colon.

  • Procedure: Sigmoidoscopy requires that you prepare by cleaning out the lower part of your colon. This usually involves consuming a clear liquid diet and using an enema (a solution you insert into your rectum) shortly before the examination. Most people do not need sedative drugs and are able to return to work or other activities the same day. During the procedure, a thin, flexible, lighted tube is inserted into the rectum and into the left side of the colon to check for polyps and cancer; the procedure may cause mild cramping. Biopsies (small samples of tissue) can be taken during sigmoidoscopy. Sigmoidoscopy may be performed in a doctor’s office.
  • Effectiveness: Sigmoidoscopy can identify polyps and cancers in the lower (descending) colon and rectum with a high degree of accuracy. Studies have shown that screening with sigmoidoscopy reduces the chances of getting or dying from colorectal cancer.
  • Risks and disadvantages: The risks of sigmoidoscopy causing a serious problem are low. The procedure could create a small tear in the intestinal wall in about 2 per every 10,000 people; death from this complication is rare. A major disadvantage of sigmoidoscopy is that it cannot detect polyps or cancers that are located only in the right side (for example, in the cecum or ascending colon to the hepatic flexure) or in the transverse colon, which are more common in older women.

Having polyps or cancers in the lower colon increases the likelihood that there are polyps or cancer in the remaining part of the colon. Thus, if sigmoidoscopy reveals polyps or cancer, colonoscopy is recommended to view the entire length of the colon.

CT colonography 

Computed tomography colonography “virtual colonoscopy” is a test that uses a CT scanner to take images of the entire colon. These images are two- and three-dimensional and are reconstructed to allow a radiologist to determine if polyps or cancers are present. The major advantages of CTC are that it does not require sedation, it is noninvasive, the entire bowel can be examined, and abnormal areas (adenomas) can be detected about as well as with traditional (optical) colonoscopy.

There are some disadvantages of CTC. Like traditional colonoscopy, CTC usually requires a “bowel prep” to clean out the colon. If an abnormal area is found with CTC, a traditional colonoscopy will be needed to see the area and take a tissue sample (biopsy)..

Stool tests 

Colorectal cancers often release microscopic amounts of blood and abnormal DNA into the stool. Stool tests can detect blood or abnormal DNA markers. Although these tests involve collecting stool samples at home, they require a prescription from a doctor.

Two types of tests, fecal occult blood testing (also called guaiac testing, or gFOBT) and fecal immunochemical testing (FIT), evaluate the stool for blood, which may be present if there is bleeding from a colorectal cancer (or other source).

If a stool test is positive, your entire colon should be examined with colonoscopy.

Stool testing reduces the risk of dying from colorectal cancer. However, because polyps seldom bleed, stool testing for blood is less likely than other screening tests to detect polyps. In addition, “false positives” are common, meaning that many people with a positive stool test will not turn out to have colorectal cancer.

A FIT-DNA test is another option and is done every three years. This test looks for specific DNA markers that may signify the presence of a colorectal cancer, and it also looks for blood in the stool. For this test, you get a special kit in order to collect a whole bowel movement. Then you follow the instructions about how and where to ship it. An abnormal test result should be followed up by colonoscopy.