What You Need to Know About Screening for Colorectal Cancer

Health Writer

Colorectal cancer is the second leading cause of cancer death in the U.S. More than 140,000 people will be diagnosed with colorectal cancer in 2018 and 90 percent of cases occur in people over 50. Screening looks for signs of colorectal cancer in people who have no symptoms. The idea behind screening for colorectal cancer is that finding cancer early will reduce the likelihood that you will die from the disease.

Who should be screened for colorectal cancer?

The U.S. Preventive Services Task Force recommends adults at average risk of developing colon cancer begin screening at age 50 and continue until age 75. After 75, you should talk to your physician about whether it makes sense to continue screening based on your health, prior screening history, and life expectancy. If you have risk factors, such as a family history of colorectal cancer, your doctor may recommend you start screening earlier. In 2018, the American Cancer Society lowered its recommended starting age to 45 for adults at average risk because of an increased incidence of colorectal cancer in people younger than 55.

Visual screening tests for colorectal cancer

Colonoscopy. “All colon cancer starts as growths called polyps,” explains Zuri Murrell, M.D., director of the Colorectal Cancer Center at Cedars-Sinai. There are multiple screening tests for colon cancer, he says. Most are good at finding cancer, but not all find polyps. “A colonoscopy is the only test that actually finds polyps. Finding a polyp allows you to remove it before it becomes cancer. So it’s a screening and prevention tool,” he says.

Before undergoing a colonoscopy, you must thoroughly remove all fecal matter from your colon so your gastroenterologist can clearly see the walls of your colon. This requires adhering to a clear liquid diet and consuming a bowel prep solution that cleans your colon.

During your colonoscopy, your physician will insert a scope with a small camera on the tip (colonoscope) into your anus and through the length of the colon, looking carefully at the walls of your colon on a computer screen to check for polyps or abnormalities. If you have polyps, your physician will remove them and send them to a lab where a pathologist will determine if they are benign (harmless) or precancerous (adenoma). Adenomas accounts for about two-thirds of polyps, and they occur in 20 to 30 percent of adults screened (the incidence of polyps increases with age). You should repeat your screening colonoscopy every 10 years, more often (usually three to five years) if you have polyps.

The biggest risk during a colonoscopy is perforation of the bowel wall. “This is very rare,” says Dr. Murrell, usually occurring less than one percent of the time. You may also experience some bleeding from removing polyps or a biopsy of abnormal tissue.

Nilofer Azad, M.D., associate professor of oncology at Johns Hopkins University School of Medicine, says a colonoscopy is considered the gold standard in colon cancer screening, catching cancer or polyps about 90 to 95 percent of the time. In the National Polyp Study, participants who had one or more adenomas removed during a colonoscopy had a lower-than-expected incidence of colorectal cancer over an average follow up of 5.9 years.

Virtual colonoscopy. The other visualization option, a virtual or computerised tomography (CT) colonoscopy, is a non-invasive screening method, Dr. Azad says. You must still clean your bowels beforehand, but your physician will use a CT scan to view your colon walls instead of a colonoscope. If you have polyps, you need to undergo a regular colonoscopy to remove them. According to the National Cancer Institute (NCI), researchers are conducting clinical trials to compare virtual colonoscopy to other colorectal cancer screening tests.

Stool tests for colorectal cancer screening

Fecal Occult Blood Test (FOBT). A yearly FOBT looks for microscopic signs of human blood in your stool. Blood in the stool may come from cancer, polyps, or some other condition. There are two tests: the guaiac FOBT and the fecal immunochemical test (FIT). For both tests, you’ll send a stool sample to a lab, which will forward the results to your doctor. According to the NCI, an annual or biannual guaiac FOBT in adults 50 to 80 years old can reduce colorectal cancer deaths by 15 to 33 percent. FIT may be better than stool guaiac at detecting adenomas. Both tests can produce false positive results, which leads to a colonoscopy.

Stool DNA test (FIT DNA). In 2014, the Food and Drug Administration approved Cologuard, the only available stool DNA test. Cologuard detects certain DNA mutations associated with colorectal cancer in addition to blood in the stool. “These tests are able to catch most people with cancer, but not everybody,” says Dr. Azad. “You’re also more likely to get a false positive. The test catches something abnormal, but you don’t know if it’s cancer.” Again, you’ll need to undergo a colonoscopy if your FIT DNA test is positive. You should repeat the FIT DNA test every three years if it’s normal.

Comparing screening tests

The gold standard for screening and preventing colon cancer is the colonoscopy, Dr. Murrell confirms. However, he says, the best test for finding cancer is whatever test patients are willing to do.

Cost may also be a factor in screening decisions as the price of the tests varies widely. According to Healthcare Blue Book, the fair price for a stool test is $52; for a screening colonoscopy, $1,728. Amazon sells a 2-pack FIT test for about $36, and Cologuard lists its price as $649. While the Affordable Health Care Act requires Medicare and private insurers to cover colorectal cancer screening, you may have a high deductible plan. Check with your insurer to see what your out-of-pocket costs will be.

Talk to your health care provider about your risks for colorectal cancer, when you should begin screening, and which test is right for you.