Your Test: Colon Cancer Screening

Colorectal cancer happens when cells that are not normal grow in your colon or rectum. These cells often form in polyps, which are small growths in the colon.

Colorectal cancer is most common in people older than 50.

Why is screening for colorectal cancer important?

Colorectal cancer usually does not cause any symptoms until it has spread. But when it has spread, it is much harder to treat.

Screening, or testing, can help find cancer or other changes early. When colorectal cancer is found early, it is much easier to treat. Testing also reduces your risk of death from colorectal cancer.

During some screening tests, your doctor also can remove polyps if needed. Not all polyps turn into cancer. But most colorectal cancer starts in a polyp.

How is screening done?

Screening tests for colorectal cancer include stool tests that can be done at home and procedures that need to be done by a doctor in a medical clinic.

  • Stool tests check your stool for signs of cancer. They include:
    • FIT (fecal immunochemical test), which checks for signs of blood in small samples of stool. This test is done every year.
    • gFOBT (guaiac fecal occult blood test), which checks for signs of blood in small samples of stool. You will need to avoid certain foods and medicines before doing this test. This test is done every year.
    • FIT-DNA (combined FIT and stool DNA), which is used to look for blood in the stool and genetic changes in DNA that could be a sign of cancer. It uses an entire stool sample. This test is done every 1 to 3 years.
  • Procedures allow your doctor to look directly at your colon. They are done less often than stool tests but require more preparation. Getting ready may include having a liquid diet for a day or two before your appointment and following instructions for cleaning out your colon. These procedures include:
    • Colonoscopy, which lets your doctor look at the inside of your entire colon. This is done every 10 years. (It will likely be done more often if you have things that increase your risk for colorectal cancer or if polyps are found.)
    • Sigmoidoscopy, which lets your doctor look at the inside of the lower part of your colon. This is done every 5 years. (Or you can have it every 10 years if you also do the FIT every year.)
    • CT colonography, also called virtual colonoscopy, which uses pictures taken during a CT scan to look at your colon. This is done every 5 years.

Colonoscopy is the only one of these tests that lets your doctor see all of your colon and at the same time remove any polyps that are found. If any of the other tests are abnormal, then you will need to have a colonoscopy.

How often should you get screened?

For people who have an average risk for colorectal cancer, regular screening is advised.

Your risk for colorectal cancer gets higher as you get older. Experts recommend starting screening at age 45 for people who are at average risk. Talk with your doctor about your risk and when to start and stop screening.

Your doctor may recommend earlier or more frequent testing if you have an increased risk, such as if you:

  • Have a close relative with a history of polyps or cancer.
  • Have inflammatory bowel disease.

What are the risks of screening?

In rare cases, a colonoscopy or sigmoidoscopy may tear the colon or cause bleeding.

Other risks include:

  • A false positive. This means that a test appears to show cancer, but there is no cancer there. If this happens, you could get more testing even though it is not needed.
  • A false negative. This means that tests do not show any cancer even though there is cancer. This may cause you to delay or not get needed treatment.

Do you have any questions or concerns after reading this information? It's a good idea to write them down and take them to your next doctor visit.