Study Shows Colonoscopy Misses 4% of Cancers

Patients Can Help Improve Accuracy of Colon Test

From the WebMD Archives

Aug. 30, 2004 -- Colonoscopy is currently considered the best way to screen for and detect colon cancer, which is the second leading cause of cancer-related deaths in the U.S. and Canada. And while the test flags the vast majority of colon cancers, it's not perfect.

Four percent of cancers on the right-hand side of the colon are not detected by colonoscopy, according to a Canadian study published in the journal Gastroenterology. Researchers specifically looked at cancer on the right side of the colon because that is where it occurs most.

It's probably not a technological flaw, say researcher Linda Rabeneck, MD, MPH, a senior researcher associated with the Cancer Quality Council of Ontario, Canada, and colleagues. More likely, the problem is less-experienced doctors, incomplete exams, and/or improper patient preparation.

In colonoscopy, patients are sedated while doctors insert a flexible, lighted tube through the rectum allowing doctors to look at the entire colon for abnormalities. The tube is attached to a video monitor and equipment which allows a biopsy to be taken.


Rabeneck and colleagues studied health records of more than 4,900 Canadians diagnosed with colon cancer between 1997 and 2001. All had had at least one colonoscopy within three years before their treatment.

In 96% of the cases, colonoscopy detected their colon cancer. But in a few patients -- 4% -- the test missed the cancer.

"It is important that we inform patients who are undergoing colonoscopy that if a cancer is present, there is a small chance that it will be missed," write the researchers.

That's no reason to cancel your colonoscopy appointment. True, few patients look forward to the exam, but it does find almost all cancers, according to Rabeneck's study.

In fact, colonoscopy's accuracy rate beats that of two other standard cancer screening tests: mammography for breast cancer and Pap smears for cervical cancer.

If you're due for a colonoscopy, you may want to look for a practitioner with extensive experience and follow any pretest instructions to the letter. Know that no test is perfect, but remember that colonoscopy is still considered the best method for detecting colon cancer while it can be treated.

WebMD Health News


SOURCES: Rabeneck, L. Gastroenterology, August 2004; vol 127: pp 452-456. The Toronto Globe and Mail. WebMD Health: "Breast Cancer: Understanding Your Mammogram Results." WebMD Health Guide A to Z: "Reliability of Pap Tests."
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