What does a positive Cologuard test mean?


Cologuard is a convenient, non-invasive, commercially-available screening test for colorectal cancer. The test is very easy for doctors to order and for patients to complete. There is no bowel prep necessary for this test, and no special diet to follow. The patient doesn’t even need to leave their house to complete the test! Sounds like a pretty good test so far…here’s how it’s done:

Once ordered, the company mails a kit to the patient’s house, and when the urge strikes, the patient places the collection device over the toilet and makes a deposit. There are a few simple preparation steps such as adding in a small amount of liquid that the company provides, and then the entire container is mailed back to the company in the provided packaging. The Cologuard test looks for certain DNA mutations and other abnormalities in the stool to determine if a test is positive or negative. A few weeks later the doctor gets a report indicating the result.

This polyp was found in the rectum of a patient referred due to a positive Cologuard test. It was removed during the colonoscopy: Pathology revealed a tubular adenoma.

This polyp (between red arrows) was found in the rectum of a patient referred due to a positive Cologuard test. It was removed during the colonoscopy: Pathology revealed a tubular adenoma.

The study that determined the characteristics of the Cologuard test basically performed the test on almost 10,000 patients at average-risk of colon cancer, and then had the patients undergo colonoscopy as the gold-standard test. The results of the Cologuard test were not available to the patients or the endoscopist at the time of the colonoscopy. The major results of this study showed that the Cologuard test had an excellent sensitivity (92%) for detecting colorectal cancer. It was far less sensitive for picking up advanced adenomatous polyps (42%). Remember that sensitivity is the main thing we care about in a screening test: we want the test to miss none of the patients who have the disease. A perfect screening test would have a sensitivity of near 100%, meaning that if 10 people have the disease out of the 1000 people screened, all 10 people would get a “positive” test result.

Sensitivity isn’t everything however…we also want a test that gives a negative result when someone does not have the disease in question. Since a full review of statistics is both 1)boring, and 2)not the point of this article, I will cut right to the chase!

What does a positive Cologuard test mean?

First and foremost, a positive result on the Cologuard test means that you need to have a colonoscopy. Not a virtual colonoscopy, or another stool test, or another scan of some sort…you need a real optical colonoscopy. Only about 4% of people will have cancer found on colonoscopy. 51% will have a precancerous polyp. The rest (45%) will have nothing found on colonoscopy. So to simplify even further, just a little more than half of people with positive results will have something abnormal (cancer or a polyp) found on colonoscopy.

What does a negative Cologuard test mean?

A negative test means that there is a less than one-percent (0.06% to be exact) chance of having cancer found on colonoscopy. However, about 34% of people with negative tests still have precancerous polyps found on colonoscopy, with the remainder (66%) of people with negative Cologuard results having truly negative colonoscopies.

What is immediately apparent from these numbers is that Cologuard rarely misses cancer. However, if we count polyps as a significant finding, there are plenty of false-positive results (45%) and plenty of false-negatives too (34%). So is Cologuard a good test overall?

I would argue that it is a great test, with one important caveat: Cologuard is not a replacement for colonoscopy! Colonoscopy is still superior to Cologuard in detecting both cancer and polyps. However, as an alternative to colonoscopy for people who either can’t or won’t get a colonoscopy, Cologuard is an excellent choice. In terms of population-based screening, I think it will get a lot more people screened overall (people who would have otherwise not done anything for screening). When compared to not screening at all, Cologuard is a whole lot better than nothing!

References:

Imperiale TF, Ransohoff DF, Itzkowitz SH, et al. Multitarget stool DNA testing for colorectal-cancer screening. N Engl J Med 2014;370:1287-97.

Cologuard website: http://www.cologuardtest.com/current-patients/how-to-use

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