FEATURE The high potential for a false posi-
tive raises even more concerns, says Douglas K. Rex, MD, chancellor’s professor and distinguished professor of medicine at the Indiana University School of Medicine and director of en- doscopy at Indiana University Hospital in Indianapolis, as well as past president of ACG.
“If the CTC detects a polyp, but
you do a colonoscopy and don’t find the lesion, you feel compelled to ex- amine the colon several times, possi- bly to examine the patient in several different positions. After that, in some instances, to repeat the CTC in order to find out if it’s really a true positive or a false positive,” Rex says. “You end up, for most of the polyp sizes, with positive predictive values that are below 50 percent. This means that scenario is going to happen a lot
in clinical practice. I believe patients will find it anxiety provoking to be told that one test found a lesion and the other test didn’t, and I think doc- tors who do colonoscopy will find it quite frustrating.” Rex also has concerns with the
Annals of Internal Medicine study’s claims that the patient experience was better with laxative-free CTC. “The way satisfaction is measured is re- ally an inadequate way to do it,” he says. “People usually go into a study like this being told that they’re going to have two tests, they’re presumably both pretty good tests and they are, afterwards, going to rate their experi- ence based on how much discomfort they had, their overall experience, etc. What the satisfaction measurements don’t take into account is the results of the study. So the question becomes
why is it that people have colonosco- py in the US? Why don’t we just have everybody have a stool occult blood test? It’s much simpler, it’s much less expensive.
“The reason people have colonos-
copy is that people have come to be- lieve, and rightfully so, that it’s the most effective test,” Rex continues. “And so the people who are answer- ing the satisfaction surveys here at the end of this study don’t actually know the results of the CTC study. If they did know the results of the study, and they knew there’s a 15 percent chance that they’re going to have a result that’s falsely positive rather than truly posi- tive, even for a large polyp, let alone the false positives that are going to oc- cur for smaller polyps, and that they’re going to have to have the test, even if it’s normal, repeated at five years rath-
18 ASC FOCUS MARCH 2013
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