Cologuard test is worth risk of false positive

DEAR DR. ROACH

>> I am a 77-yearold man in good health. At my recent annual physical exam, my physician ordered a Cologuard kit to screen for colorectal disease.

I never had problems with my gastrointestinal tract.

I had two colonoscopies about several years ago, the results 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 followup colonoscopy, which is also riskier for elderly patients.

Should I do this? 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, Would you agree?).

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, so it can serve as a warning signal.

Contact Dr. Roach at ToYourGoodHealth@med. cornell.edu.