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   Home      Current Topics      Why the Cologuard Test Should Replace Colonoscopy

The FDA has approved (August 11, 2014) Cologuard for the detection of colon cancer and the presence of pre-cancerous polyps. The Cologuard test, which like the FIT test (fecal immunochemical test), involves simply an at-home collection of feces, screens for blood and sDNA associated with cancerous tissue and yields an accuracy of 92% when compared with colonoscopy [1], [2], [3]. Put differently, colonoscopy apparently detects ca. 8% more cancerous or pre-cancerous tissue than Cologuard. That would seem to make Cologuard competetive, but not equal  to colonoscopy in utility as a screening procedure for colon cancer. That's if you really believe a study involving humans can have an accuracy such that 8 % is meaningful (think of all the studies, such as the one in which vitamin E was supposed to save us from heart attacks and then it was found in a later study to do exactly the opposite, etc.)

But that is not the whole story: when a patient gets screened for colon cancer, what are they trying to achieve? They are trying to avoid serious illness; and therefore the calculation of must include the chance of serious complications from the screening procedure itself, for example, if there was a colon cancer screening procedure that was 100% accurate but resulted in 20% of the screened patients dying, would you be willing to take the test? Of course not: you would recognize that the testing involved more risk that the condition it screened for; the total screening-testing accuracy calculation would be in favor of avoiding the test. Let's now apply the same logic to colonoscopy versus Cologuard in terms of avoiding serious illness:

As an informed, but approximate figure, let's say that colonoscopy is 65% accurate when it comes to screening for colon cancer [4] and that 1 in 200 colonoscopies result in serious complications [5]. The combined risk to the patient is therefore:


35% (the chance the colonoscopy missed the cancer) times the chance of serious complications (1 in 200, or 0.005) = 0.18%


43% (the chance the Cologuard missed the cancer is about 8% higher than colonoscopy) times the chance of serious complications* (1 in 1000?-1 in 10, 000?) = 0.043% max

[ *I need you, dear reader, to use your imagination to figure out what could possibly go wrong that would cause a patient to sustain a serious injury while collecting a stool sample and what the odds of that event might be. I thought of slipping on the bathroom floor and falling head-first into the toilet, but alas, that is not a test-specific risk; one can fall head-first into the toilet while minding one's business).

The conclusion of the calculation is that you are a minimum of four times-and it may be 40 or 400 times -more likely to suffer serious injury, illness, or death by choosing colonoscopy versus Cologuard as a screening test.

Moral of the Story: all factors must be considered when doing a calculation of risk.  Choose wisely.