A Canadian study published in the Annals of Internal Medicine had concluded that the common test, which scans the inside of the colon for cancers is associated with fewer deaths from colorectal cancer just because they usually form on the left side of the organ.

The goal of this study was to examine the association between colonoscopy and colorectal cancer deaths in a cohort from Ontario, Canada, of persons aged 52 to 90 years who were diagnosed with CRC from January 1996 to December 2001 and who died of CRC by December 2003.
In the new study, the test missed just about every cancer in the right side of the colon, where cancers are harder to detect but is where about 40 percent arise. And it also missed roughly a third of cancers in the left side of the colon.

The authors speculate some colonoscopies billed as "complete" may not actually be looking at the total right colon. Alternatively, it could be that bowel preparation isn’t as good in the right colon, making it more difficult for doctors to spot pre-cancerous polyps, or polyps in the right colon might be flatter and thus more difficult to see and remove during the test.

Dr. David F. Ransohoff, a gastroenterologist at the University of North Carolina, and other screening experts say patients should continue to have the test, adding that it is still highly effective. But they also recommend patients seek the best colonoscopists by, for example, asking pointed questions about how many polyps they find and remove.