Cologuard – What Is It?

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November 5, 2014

Recently Cologuard has been in the news.

What is it? Cologuard is a Medicare/Medicaid approved stool analysis which aims to detect alterations in the DNA associated with colon cancer and precancerous polyps. It combines analysis for a variety of abnormal patterns in your DNA with a fecal immunochemical test or FIT, which assesses for blood in the stool that may not be visible to the naked eye. In the past, your doctor may have done a version of the FIT test by placing the stool from a rectal exam on a card or in a container.

Most of the DNA in our stool comes from bacteria that live in your gut. It is estimated that only 0.01% comes from our own cells that are shed by our intestinal lining. Cologuard aims to detect the less than 0.01% of abnormal DNA from colon cancer or precancerous polyps.

What is involved in doing this test? When it is ordered by your doctor, a stool collection kit is sent to you in the mail. You do not need to alter your diet, medications, or take a laxative. You do not need to go to a lab to complete the test. You mail it back to Exact Sciences in a prepaid envelope and they do the analysis and give you a result of positive (abnormal) or negative (normal) within approximately 2 weeks. If the test is positive, you should then have a colonoscopy.

What does a positive Cologuard test mean? Let’s look at the sensitivity of the test – the likelihood that the test will be positive when a person has a colon cancer or a polyp which can potentially lead to cancer. A study in the NEJM looked at 9989 patients who either did Cologuard or FIT. It was found that the Cologuard test was 92% sensitive for detecting colon cancer, 69% sensitive for detecting high grade dysplasia/HGD (polyps that are very close to becoming cancer), and 42% sensitive for detecting sessile serrated adenomas/SSA greater than 1cm in size (a marker of increased risk for colon cancer).

This compared favorably with the FIT test which had a sensitivity of 74% for cancer, 46% for HGD and 5% for SSAs. It makes sense that Cologuard is more sensitive than FIT alone since Cologuard combines the FIT with a test for genetic changes. The false positive rate was 13% meaning 13/100 patients screened as positive, had no significant findings when a follow-up colonoscopy was done. The false positive rate of the FIT was significantly lower at only 5%. If we combine these statistics, it means that the FIT alone would miss more colon cancers than Cologuard would, but would also lead to fewer normal follow-up colonoscopies if we were to depend on these tests to determine when to recommend a colonoscopy.

The CEO of Exact Sciences said in a Wall Street Journal interview that if their test is positive, which in the company’s experience happens 16% of the time, there is a 4% chance that you will have colon cancer, a 20% chance you will have a precancerous polyp and a 76% chance you’ll have either a benign polyp or no polyp. The last category accounts for the ‘false positives’.

Who should do this test? Colonoscopy remains the gold standard for colon cancer screening and naturally follows when this test is positive. Cologuard is approved by Medicare for patients between the ages of 50 and 85 who are ASYMPTOMATIC and at AVERAGE risk for colon cancer. Those who have a family history of colon cancer, a personal history of precancerous polyps, ulcerative colitis or colon cancer should be screened with colonoscopy. The test is aimed at those that have not had a colonoscopy due to fear, high risk, etc. This amounts to about 23 million Americans.  By screening more people, we would hope to avoid many cases of advanced colon cancer that are detected only after people start to have symptoms of bleeding or colon obstruction, and are less likely to respond to treatment.

Currently the test is covered by Medicare which reimburses $502 for it. The out of pocket expense for Cologuard to those without insurance is $599. This is the amount that would be charged to your private insurer. The test is recommended to be done every 3 years as opposed to FIT which should be done annually and costs $25. FIT does not assess for abnormalities in DNA.  MDVIP, a network of concierge primary care physicians has come out in support of this study and has partnered with Exact Sciences. It is hoped that the availability of Cologuard will facilitate screening for those of you who have hesitated to have a colonoscopy.

Colonoscopy remains the gold standard. I would recommend that you talk to your doctor about colon cancer screening as it truly saves lives.


Imperiale, Thomas F. et al, “Multitarget Stool DNA Testing for Colorectal Cancer Screening” NEJM 2014; 370-1287-97