Ask the doctors: Colonoscopies best for detecting colon cancers

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Q: I know that colonoscopies are more thorough than the at-home DNA stool test for colon cancer. But how much better? I have such a terrible time with anesthesia and gas. Both make me nauseated for several days. No one in my family has ever had any colon problems or polyps. I'd really like to pass, if possible.

A: As we discussed in a previous article, although colonoscopy remains the gold standard in colon cancer screening, it does indeed pose the risk of complications, leading some people to skip the procedure. If you have problems with anesthesia — which isn't required but makes the procedure more tolerable — I can understand your additional worry. As for the DNA stool test, known as Cologuard, it's relatively new (approved by the FDA in 2014), but can provide an option in some cases.

Let's take a look at how the test works. It helps to understand that colon cancer arises from a series of genetic changes and alterations in how genes transmit their information. An effective way to detect these genetic markers in stool tests had been unsuccessful until the advent of Cologuard. The test looks at four molecular DNA markers in addition to a highly specific test for hemoglobin in the stool.

A 2014 study published in the New England Journal of Medicine compared the Cologuard test with FIT screening (a test that simply looks for hemoglobin in the stool); 9,989 people had both of the tests and then had a colonoscopy 90 days later. Of the 9,989, 65 people were found to have colon cancer. FIT detected about 73.8 percent of these cancers, while Cologuard detected 92.3 percent. That sounds good, doesn't it? But when it comes to advanced precancerous polyps, Cologuard detected 42.4 percent, while FIT detected only 23.8 percent. For non-cancerous polyps, Cologuard detected only 17.2 percent, while FIT detected 7.6 percent.

The problem in not being able to accurately detect non-cancerous polyps is that such polyps can develop into cancer that goes undetected until the next time you screen for colon cancer, by which point it may be advanced. Currently, the recommendation for Cologuard is to test every three years. Although you could conceivably test more frequently to detect precancerous lesions as they become cancerous, Cologuard has a high false-positive rate of 16.1 percent.

Here's why that matters in practical terms: Each positive result with Cologuard means a patient then must undergo a colonoscopy. So high rates of false positives ultimately lead to more colonoscopies.

Cologuard is a good test for colon cancer, but its inability to find non-cancerous colon polyps, its high false-positive rate and its cost ($599) prevent it from being a great test. It would be a good choice for people who simply cannot undergo a colonoscopy. That includes people with illnesses that increase the complication rate of colonoscopy, such as severe diverticulitis, diabetes, heart failure and cancer, and people over the age of 75.

Remember, you don't have to use anesthesia to undergo a colonoscopy. If you can tolerate the procedure, I would still recommend colonoscopy. It's the best colon cancer screening test for a healthy person.



Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles. Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.

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