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CMAJ. 2016 September 20; 188(13): 973.
PMCID: PMC5026517

Colonoscopy as best screening test not proven

Iain Murray, MD CM MSc

The Canadian Task Force on Preventive Health Care recommends testing of fecal occult blood or flexible sigmoidoscopy for patients at average risk of colon cancer.1 The guideline states that there is not enough evidence for colonoscopy as a primary population-based screening test. This does not mean that the task force thinks colonoscopy will not save lives; it means only that, to date, there are no randomized controlled trials that prove it.

Colonoscopy is considered the gold standard for colon investigation. Indeed, if anything is found on fecal occult blood or by sigmoidoscopy, patients are referred for colonoscopy. It is the test advised for higher risk patients with a family history of colorectal cancer.2 Randomized control trials for colonoscopy as the primary screening tool have not been done in the United States, because it was considered unethical to assign people randomly to not to receive a colonoscopy. There are trials underway in Canada, Europe and the US, but it will be years before the results are available. There are many case–control studies showing a clear benefit of colonoscopy.3 The concern is that, while we are waiting for prospective studies, patients may be denied a test that will likely save more lives than fecal occult blood tests or flexible sigmoidoscopy.

As gastroenterologists, we would rather find polyps, the source for most cancers. Colonoscopy is able to detect and remove polyps so that cancer does not develop in the first place. The other tests find only some colon cancers, and certainly miss others, especially early ones.

The task force suggests that colonoscopy does harm.1 The 1:1000 published perforation rate during colonoscopy (about three times less in Ontario) is more likely to occur if a therapeutic manoeuver, such as polypectomy, has taken place. This is done in about one-third of cases. Because the incidence of colon cancer (1:19) far outweighs risk associated with colonoscopy, we are concerned that there could be more harm done when cancers are missed by inferior tests.

In the end, all patients should discuss these issues with their family doctors. Although we appreciate that there may be long wait lists for colonoscopy, there are some parts of Canada where colonoscopy is more readily available, and it should be discussed as one of the options.

It is important that all those aged between 50 and 75 are screened for colon cancer. Screening should be done earlier if there is a family history of colon cancer. Doing any test is better than doing nothing at all. Symptoms such as bleeding, change in bowel habits, abdominal pain or weight loss need to be investigated at any age. Time will tell if the best screening test is colonoscopy.


1. Canadian Task Force on Preventive Health Care. Recommendations on screening for colorectal cancer in primary care. CMAJ 2016;188:340–8. [PMC free article] [PubMed]
2. Winawer S, Fletcher R, Rex D, et al. ; Gastrointestinal Consortium Panel. Colorectal cancer screening and surveillance: clinical guidelines and rationale — update based on new evidence. Gastroenterology 2003;124:544–60. [PubMed]
3. Pan J, Xin L, Ma YF, et al. Colonoscopy reduces colorectal cancer incidence and mortality in patients with non-malignant findings: a meta-analysis. Am J Gastroenterol 2016;111:355–65. [PMC free article] [PubMed]

Articles from CMAJ : Canadian Medical Association Journal are provided here courtesy of Canadian Medical Association