Testing matters. The difference between colon cancer screening, prevention | Opinion
Colon cancer (colorectal cancer) remains one of the most common and deadly cancers. It is now the leading cause of cancer deaths in Americans under age 50, and the second leading cause of cancer deaths in non-smokers over the age of 50, according to the American Cancer Society. Current guidelines list several options as acceptable for colon cancer screening, yet an important distinction has become blurred: the difference between cancer screening and cancer prevention. Patients and even some healthcare providers misunderstand what currently available tests do.
I recently saw an advertisement for yet another company promoting their colon cancer detecting blood test, reporting only “22% sensitivity for advanced adenomas.” This means it misses nearly 80% of very important pre-cancerous polyps. (“Sensitivity” is a test’s ability to correctly identify a condition, similar to accuracy. An “adenoma” is a pre-cancerous polyp growth, and “advanced adenomas” carry higher cancer risk).
The problem is that patients and some healthcare providers don’t understand how limited these tests currently are for prevention of colon cancer, leading many to believe these are equivalent to a colonoscopy. Those boxes that come in the mail for collecting your stool (e.g. Fecal Immunochemical Tests — FIT — and Cologuard), as well as other similar blood tests, though better than nothing at all, are cancer detection tests, not cancer prevention tests.
Stool-based tests like FIT have an important limitation. FIT relies on detecting blood in stool, meaning abnormalities are detected only once bleeding occurs. However, polyps usually do not bleed until they are troublesome, and even early stage colon cancer may not bleed.
Cologuard performs better than FIT alone, but still has important limitations for prevention. A negative Cologuard result is reassuring that cancer is unlikely at this moment (99% negative predictive value), but its ability to detect “advanced adenomas” is poor (42%). And for detection of polyps that are soon to become cancer (“advanced adenomas with high-grade dysplasia”) it misses 31% (e.g. 69% sensitivity), according to information on their website and data published in the pivotal New England Journal of Medicine study. A missed high risk polyp can continue to grow silently. In these cases, the test may be technically “right” at the moment about not having colon cancer, but the opportunity for cancer prevention may be missed. Furthermore, the “2 steps” (FIT plus DNA) use an algorithm that does not indicate whether the FIT or DNA component triggered the alert for possible cancer risk.
Colonoscopy detects and removes polyps, including small ones, and according to data cited by the US Preventive Services Task Force in JAMA 2021, identifies 89-95% of advanced adenomas, which can be removed during the same procedure for cancer prevention. And these detection numbers are only improving with the use of better colonoscope camera optics and AI detection tools nowadays. Other non-invasive tests may detect some problems, but still require follow-up colonoscopy and may miss important growths. This is why colonoscopy is referred to as the “gold standard” (best test).
When a FIT or Cologuard is abnormal, colonoscopy is the next step. In some cases, this approach can delay diagnosis by months or years from when an abnormality would have ideally been identified.
Some people are deterred because they have heard horror stories about bowel preparation. Modern colonoscopy prep has gotten much easier. Many patients can eat a modified diet and continue normal activities the day before, while using low-volume or tablet-based prep options. Colonoscopy may be inconvenient and require planning, but the consequence and therapy for cancer is far more burdensome.
Colonoscopy reduces colorectal cancer occurrence and deaths by finding and removing pre-cancerous polyps before they become cancer. Current stool tests and blood tests can detect some colon cancers, but their ability to help prevent cancer is presently limited.
As March’s “Colorectal Cancer Awareness Month” approaches, it is a good time to understand the difference. For most average-risk adults, prevention starts at age 45, earlier with family history or other risk factors. If the goal is prevention — not just detection — the choice of test matters. Colonoscopy remains the most effective tool to find and remove pre-cancerous growths in advance of their progression to cancer.
Dr. Todd Witte is a board-certified gastroenterologist who has practiced in Bellingham for nearly 19 years and serves as medical director at Northwest Gastroenterology. Having cared for patients affected by colorectal cancer, he is committed to promoting screening and prevention.