Test worth false-positive risk

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August 5, 2019 - 9:53 AM

Dear Dr. Roach: I am a 77-year-old man in good health. At my recent annual physical exam, my physician ordered a Cologuard kit to be sent to me to screen for colorectal disease. I have never had symptoms of disease or problems with any part of my gastrointestinal tract. I had two colonoscopies at different times several years ago, the results of which were negative. When I got the kit, I read the patient guidelines and learned that the Cologuard test could result in false positives and that there is an unspecified increase in the risk of false positives for persons over the age of 75. A positive result from the test requires a follow-up colonoscopy, which is also riskier for elderly patients. Since I am in good health with no symptoms of any disease, I decided I did not want to risk a false positive result from the test, which would require a follow-up colonoscopy. Did I make the right decision? — E.G.

Answer: Cologuard is a brand of test that looks at DNA in stool for evidence of cancer. Certain DNA mutations are highly suggestive of cancer. In addition, Cologuard does an immunochemical test looking for blood. Because it combines these two techniques, Cologuard is more sensitive than only a test looking for blood. Compared with colonoscopy, Cologuard is about 92% sensitive in finding cancers, meaning it will miss about 8%. With improved sensitivity comes a lack of specificity, which means you could have a false positive result. The Cologuard can say that you have an abnormality when a colonoscopy would indicate normal. For 45% of people with a positive Cologuard test, no abnormalities will be found upon colonoscopy. You are right that false positives are more prevalent in older individuals.

There are two reasons that I think the Cologuard is still a reasonable choice, despite the fact that it isn’t as good as colonoscopy. The first is that I continue to recommend a screening test for healthy 77-year-old men, and although colonoscopy is my first choice, a normal Cologuard test prevents a need for colonoscopy, and a false positive would lead to the same colonoscopy you would have gotten anyway (although there probably is more anxiety after a false positive test). The second is that a person with an abnormal Cologuard and a normal colonoscopy may be at risk for an abnormality that wasn’t found on colonoscopy. A study looking at people in this category found a 25% risk of abnormality on follow-up colonoscopy, if the Cologuard was abnormal again one to two years later.

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