Choose the right colon cancer screening test

March is Colorectal Cancer Awareness Month. Approximately 140,000 people in the United States will be diagnosed with colon cancer this year. The lifetime risk of developing colon cancer is 1 in 20 and only lung cancer claims more lives each year.

As gastroenterologists, we spend a significant amount of our time and effort working to prevent, diagnose, and treat colon cancer. Because of improved compliance with colon cancer screening, the number of new colon cancer diagnoses and deaths have both decreased by more than 30% over the last 20 years. It is truly one of the success stories in modern medicine.

At the center of this dramatic decline in mortality is colonoscopy. Colonoscopy is the gold standard for both the prevention and diagnosis of colon cancer. The procedure is not only safe and highly accurate, but unlike any other screening test, it allows for the identification and removal of polyps, which are the precursors of colon cancer. Removal of polyps is the main reason that the number of colon cancer diagnoses and deaths are decreasing.

In the last few years, a new test has emerged for colon cancer screening. Stool DNA testing has been combined with testing for microscopic amounts of blood in the stool. This test has been marketed by Exact Sciences as Cologuard® and more than 1 million people have utilized this type of screening.

Unfortunately, there are significant limitations to stool DNA tests. In our experience at Rockford Gastroenterology Associates, many patients who have completed stool DNA testing are unaware of these limitations. Many gastroenterologists are concerned that increased use of this test will reverse some of the gains we have made against this deadly disease.

As an informed patient, there are important facts that you need to consider in order to make an educated decision regarding which colon cancer screening test is right for you.

First, stool DNA testing has limited sensitivity for the identification of colon polyps. For advanced polyps 10 millimeters or larger, the sensitivity is less than 50%. That means if you have an advanced polyp with a high risk of progression to colon cancer, this test will detect it less than half the time. For patients who have already developed colon cancer, the test will miss 8% of cases. These numbers are unacceptable given the high accuracy of screening colonoscopy.

Second, stool DNA testing has a significant false positive rate. The initial studies suggested that stool DNA testing has a false positive rate of 14% (normal colonoscopy after abnormal stool DNA test). In our clinical experience, that number has been higher. When combined with the miss rate, stool DNA testing has rather poor accuracy.

Finally, it is important to understand how most medical insurance companies, including Medicare, handle payment for colon cancer screening. The first colon cancer screening test is generally covered, and this includes both colonoscopy and stool DNA testing. However, if you have a positive stool DNA test and then receive a follow-up colonoscopy, you may be responsible for a greater portion of the cost than if you utilized colonoscopy as the first test.

Based on these limitations, the United States Multi-Society Task Force consensus guidelines from 2017 list Cologuard® as a Tier 2 test (colonoscopy is Tier 1). The American College of Physicians guidelines from 2019 do not list Cologuard® amongst the recommended colon cancer screening tests. Another study from 2019 concluded that it was less effective and more costly than other colon cancer screening tests covered by Medicare.

Armed with this information, you can have meaningful discussions with your primary care provider and choose the colon cancer screening test that is right for you.

Dr. Aaron Shiels is a gastroenterologist and managing partner at Rockford Gastroenterology Associates.