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Fact Sheet

  • Reviewed: 12/19/2013

Tests to Detect Colorectal Cancer and Polyps

Key Points

  • Expert groups recommend that people at average risk for colorectal cancer start regular screening at age 50.
  • Current colorectal cancer screening tests check for blood in stool (high-sensitivity fecal occult blood tests) or use an instrument to look at the lining of the colon and rectum (sigmoidoscopy and colonoscopy).
  • People should talk with their health care provider about when to begin screening for colorectal cancer, what test(s) to have, the advantages and disadvantages (including potential harms) of each test, and how often to undergo screening.
  • New methods of screening for colorectal cancer, such as virtual colonoscopy and tests that analyze human DNA in stool or blood samples for certain changes, are under study.
  1. What is colorectal cancer?

    Colorectal cancer is a disease in which abnormal cells in the colon or rectum divide uncontrollably, ultimately forming a malignant tumor. (The colon and rectum are parts of the body’s digestive system, which takes up nutrients from food and water and stores solid waste until it passes out of the body.)

    Parts of the colon. Drawing of the front of the abdomen that shows the four sections of the colon: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. Also shown are the small intestine, the cecum, and the rectum. The cecum, colon, rectum, and anal canal make up the large intestine. The cecum, ascending colon, and transverse colon make up the upper, or proximal, colon; the descending colon and sigmoid colon make up the lower, or distal, colon.

    Most colorectal cancers begin as a polyp, a growth in the mucous tissue that lines the inner surface of the colon or rectum. Polyps may be flat, or they may be raised. Raised polyps may grow on the inner surface of the colon or rectum like mushrooms without a stalk (sessile polyps), or they may grow like a mushroom with a stalk (pedunculated polyps).  Polyps are common in people older than 50 years of age, and most are benign (noncancerous). However, a certain type of polyp known as an adenoma may develop into cancer.

    Colorectal cancer is the third most common type of non-skin cancer in both men (after prostate cancer and lung cancer) and women (after breast cancer and lung cancer). It is the second leading cause of cancer death in the United States after lung cancer. Although the rates of new colorectal cancer cases and deaths among adults aged 50 years or older are decreasing in this country, incidence is increasing among younger adults (1). It is estimated that in 2014, a total of 136,830 people in the United States will be diagnosed with colorectal cancer and 50,310 people will die from it (1).

    Although the major risk factor for colorectal cancer is older age, several other factors have been found to be associated with increased risk, including excessive alcohol use, obesity, being physically inactive, cigarette smoking, and possibly diet. In addition, people with a history of inflammatory bowel disease (such as ulcerative colitis or Crohn disease) have a higher risk of colorectal cancer than people without such conditions. And people who have a family history of colorectal cancer or certain inherited conditions (such as Lynch syndrome and familial adenomatous polyposis) also have an increased risk of colorectal cancer. (More information about risk factors for colorectal cancer is available in NCI’s PDQ summary on Colorectal Cancer Screening.)  

    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.

  2. 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).
    • Sigmoidoscopy: 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).

    • Other methods: 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).

  3. How can people and their health care providers decide which colorectal cancer screening test(s) to use? 

    People should talk with their health care provider about when to begin screening for colorectal cancer, what test(s) to have, the advantages and disadvantages of each test, how often to undergo colorectal cancer screening, and when to stop.

    The decision about which test to have usually takes into account several factors, including:

    • The person’s age, medical history, family history, and general health
    • The potential harms of the test
    • The preparation required for the test
    • Whether sedation may be needed for the test
    • The follow-up care needed after the test
    • The convenience of the test
    • The cost of the test and the availability of insurance coverage

    The commonly used colorectal cancer screening tests all have advantages and disadvantages:

    Fecal Occult Blood Test (guaiac FOBT or fecal immunochemical test)


    • No cleansing of the colon is necessary.
    • Samples can be collected at home.
    • Cost is low compared with other colorectal cancer screening tests.
    • There is no risk of damage to the lining of the colon.
    • No sedation is needed.


    • The test does not detect some polyps and cancers.
    • False-positive test results (i.e., the test suggests an abnormality when none is present) are possible.
    • Dietary restrictions are needed before guaiac FOBT.
    • Additional procedures, such as colonoscopy, may be needed if the test result shows blood in the stool.



    • For most patients, discomfort is minimal, and complications are rare.
    • The doctor may be able to perform a biopsy or polypectomy (removal of a polyp or adenoma) during the test, if necessary.
    • Less extensive cleansing of the colon is necessary for this test than for a colonoscopy.
    • Sedation is often not required.


    • Abnormal growths in the upper part of the colon will be missed because the test allows the doctor to view only the rectum and the lower part of the colon.
    • Bowel cleansing is needed before the test.
    • Medication and diet changes may be needed before the test.
    • There is a very small risk of bleeding or of tearing or perforation of the lining of the colon.
    • Additional procedures, such as colonoscopy, may be needed if the test finds an abnormality.
    • The availability of sigmoidoscopy has decreased substantially in the United States in recent years (10).

    Standard Colonoscopy


    • This test is one of the most sensitive currently available.
    • It allows the doctor to view the rectum and the entire colon.
    • The doctor can perform a biopsy or polypectomy during the test if necessary.


    • Even though this test is highly sensitive, it still may not detect all small polyps, nonpolypoid lesions, or cancers.
    • A thorough cleansing of the colon is required before the test.
    • Diet changes are needed before the test, and medications may need to be adjusted.
    • Some form of sedation is almost always used. As a result, the patient must have someone accompany them to the procedure and drive them home afterward, and they may not be able to work the day of the procedure.
    • There is a small risk of bleeding or of tearing or perforation of the lining of the colon; this risk increases with age, the presence of other health problems, and when polyps are removed.

  4. Does health insurance pay for colorectal cancer screening?

    The Affordable Care Act requires coverage of colorectal cancer screening tests by health plans that started on or after September 23, 2010. (For health plans that started before September 23, 2010, the rules about insurance coverage are covered by state laws, which vary, and by other federal laws.) People should check with their health insurance provider to determine their colorectal cancer screening coverage. Medicare covers several colorectal cancer screening tests for its beneficiaries. Specific information about Medicare benefits for colorectal cancer screening is available on the Medicare website.

  5. What happens if a colorectal cancer screening test finds an abnormality?

    If a screening test finds an abnormality, additional tests may be recommended. These tests may include x-rays of the gastrointestinal tract, a sigmoidoscopy, or, most often, a colonoscopy.  If a polyp or tumor is found during a sigmoidoscopy, a biopsy or polypectomy may be performed during the test, and a colonoscopy may be recommended. If a polyp or tumor is found during a standard colonoscopy, a biopsy or polypectomy may be performed during the test to determine whether cancer is present. If a polyp or tumor is detected during virtual colonoscopy, the patient will be referred for a standard colonoscopy.

  6. What new tests are being developed for colorectal cancer screening?

    Researchers are studying whether DNA in stool (4, 11, 12) or blood (13) can be tested to screen for colorectal cancer. DNA is normally present in stool as a result of cells being shed from the lining of the colon and the rectum. Testing DNA in stool for certain changes may help doctors find evidence of colorectal cancer or even precancerous growths. Research conducted thus far has shown that this kind of test has the potential to detect colorectal cancer in people who are known to have the disease, but whether this type of test can be used for screening (that is, to detect colorectal cancer in people who do not have symptoms) is not known. Tumors also release cells, and therefore DNA, into the bloodstream, and researchers are studying whether the presence of an altered gene called SEPT9 in blood can be used to screen for early cancer and advanced adenomas (13).

    In addition, several approaches that avoid the thorough cleansing of the colon that is currently required for virtual colonoscopy are being studied and refined. One approach is "fecal tagging" with a contrast agent that is ingested over several days before the procedure. This technique allows fecal material in the colon to be differentiated from colon tissue (14); computer software can be used to electronically remove the tagged fecal material from images (15).

    Information about ongoing clinical trials that are studying methods for colorectal cancer screening can be found in NCI’s clinical trials database. You may also contact NCI’s Cancer Information Service at 1–800–4–CANCER (1–800–422–6237) for assistance with searching the clinical trials database or for other cancer information needs.

Selected References
  1. American Cancer Society. Cancer Facts & Figures 2014. Atlanta: American Cancer Society; 2014. Accessed February 25, 2014.

  2. U.S. Preventive Services Task Force. Screening for colorectal cancer: U.S. Preventive Services Task Force recommendation statement. Annals of  Internal Medicine 2008; 149(9):627-637.

    [PubMed Abstract]
  3. Burch JA, Soares-Weiser K, St John DJ, et al. Diagnostic accuracy of faecal occult blood tests used in screening for colorectal cancer: A systematic review. Journal of Medical Screening 2007; 14(3):132-137.

    [PubMed Abstract]
  4. Ouyang DL, Chen JJ, Getzenberg RH, Schoen RE. Noninvasive testing for colorectal cancer: A review. American Journal of Gastroenterology 2005; 100(6):1393-1403.

    [PubMed Abstract]
  5. Elmunzer BJ, Hayward RA, Schoenfeld PS, et al. Effect of flexible sigmoidoscopy-based screening on incidence and mortality of colorectal cancer: A systematic review and meta-analysis of randomized controlled trials. PLoS Medicine 2012; 9(12):e1001352.

    [PubMed Abstract]
  6. Schoen RE, Pinsky PF, Weissfeld JL, et al. Colorectal-cancer incidence and mortality with screening flexible sigmoidoscopy. New England Journal of Medicine 2012; 366(25):2345-2357.

    [PubMed Abstract]
  7. Atkin WS, Edwards R, Kralj-Hans I, et al. Once-only flexible sigmoidoscopy screening in prevention of colorectal cancer: a multicentre randomised controlled trial. Lancet 2010; 375(9726):1624-1633.

    [PubMed Abstract]
  8. Ransohoff DF. How much does colonoscopy reduce colon cancer mortality? Annals of Internal Medicine 2009; 150(1):50-52.

  9. Collins JF, Lieberman DA, Durbin TE, Weiss DG; Veterans Affairs Cooperative Study #380 Group. Accuracy of screening for fecal occult blood on a single stool sample obtained by digital rectal examination: A comparison with recommended sampling practice. Annals of Internal Medicine 2005; 142(2):81-85.

    [PubMed Abstract]
  10. Zapka J, Klabunde CN, Taplin S, et al. Screening colonoscopy in the US: Attitudes and practices of primary care physicians. Journal of General Internal Medicine 2012; 27(9):1150-1158.

    [PubMed Abstract]
  11. Imperiale TF, Ransohoff DF, Itzkowitz SH, et al. Fecal DNA versus fecal occult blood for colorectal-cancer screening in an average-risk population. New England Journal of Medicine 2004; 351(26):2704-2714.

    [PubMed Abstract]
  12. Itzkowitz SH, Jandorf L, Brand R, et al. Improved fecal DNA test for colorectal cancer screening. Clinical Gastroenterology and Hepatology 2007; 5(1):111-117.

    [PubMed Abstract]
  13. Church TR, Wandell M, Lofton-Day C, et al. Prospective evaluation of methylated SEPT9 in plasma for detection of asymptomatic colorectal cancer. Gut 2014; 63(2):317-325.

    [PubMed Abstract]
  14. Iannaccone R, Laghi A, Catalano C, et al. Computed tomographic colonography without cathartic preparation for the detection of colorectal polyps. Gastroenterology 2004; 127(5):1300-1311.

    [PubMed Abstract]
  15. Zalis ME, Blake MA, Cai W, et al. Diagnostic accuracy of laxative-free computed tomographic colonography for detection of adenomatous polyps in asymptomatic adults: A prospective evaluation. Annals of Internal Medicine 2012; 156(10):692-702.

    [PubMed Abstract]