Colonoscopies are increasingly being performed on adults, even those without warning signs of cancer, because it is the most effective way to screen for colorectal cancer. Still, the colonoscopy can miss dangerous polyps, especially when performed by clinicians who do not have much experience. Canadian scientists decided to look at the approximate rate at which colonoscopies missed markers of cancer.

What the researchers wanted to know: How often do colonoscopies miss polyps or other markers of colorectal cancer?

What they did: The researchers used information from several databases to find a group of 4,920 patients who had surgery for colorectal cancer on the right side of the colon, the side closest to the small intestine. They used the right side of the colon because it is farthest inside the body, and so therefore is the most often missed when a colonoscopy is incomplete. They correlated the hospital admission date of these patients with the date that they had their most recent colonoscopy. A colonoscopy within six months of the hospital admission date counted as a detection of cancer; if the colonoscopy was prior to six months before hospital admission, the researchers counted it as having missed the cancer.

What they found: Out of the 4,920 patients with right-sided colorectal cancer, 2,654 (54 percent) had a colonoscopy within three years of their hospital admission date. In patients who had a colonoscopy, 96 percent had the most recent colonoscopy within six months before they were admitted to the hospital for cancer, so the researchers assumed that the colonoscopy found evidence of cancer. That left only 105 patients, 4 percent, for whom the colonoscopy missed signs of cancer.

What it means to you: This study shows that even when people are visiting a regular clinic, colonoscopies still detect cancer most of the time. Other studies on the success rate of colonoscopies have concentrated on academic centers and used endoscopists with lots of experience, and though they have found similar success rates, researchers worried those rates were inflated. While there is a small chance that a colonoscopy will miss a potential malignant growth, odds are it will catch cancer if it is there.

Caveats: The formula that the researchers used may have skewed the success rate of colonoscopies. On one hand, they could have overestimated its effectiveness by looking only at patients' most recent colonoscopy and not taking into account previous colonoscopies that also may have missed the cancer. On the other hand, some cancers develop quite quickly, and those patients may have had a truly normal colonoscopy six months or less before hospital admission.

Find out more: An explanation of the uses of a colonoscopy and how it works can be found through the National Institutes of Health.

The American Society for Gastrointestinal Endoscopy can also be a good resource, especially if you have already decided you want screening and want to know what to expect or how to choose a doctor.

Read the article: Bressler, B. et al. "Colonoscopic Miss Rates for Right-Sided Colon Cancer: A Population-Based Analysis." Gastroenterology. August 2004, Vol. 127, No. 2, pp. 452–456.

Abstract online: www2.gastrojournal.org