Colonoscopy is a reasonably effective way of finding and removing colon polyps, the benign lesions from which most colon cancers eventually develop. The U.S. Preventive Services Task Force has found what it judges to be "fair to good evidence" that several screening methods, including colonoscopy, are effective in reducing mortality from colon and rectal cancer. It has concluded that the benefits from screening substantially outweigh potential harms, but the quality of evidence, magnitude of benefit, and potential harms vary with each method.

The Task Force recommends initiating colonoscopy screening at 50 years of age for men and women at average risk for colorectal cancer. But in persons at higher risk (for example, those with a first-degree relative who receives a diagnosis of colorectal cancer before 60 years of age), initiating screening at an earlier age is reasonable. As a general rule, people over age 50 should undergo colonoscopy once every five years. If a doctor has previously found and removed a polyp, that schedule is moved up to a three year interval.

But now new concerns have emerged over the accuracy of many colonoscopy examinations. The ability of colonoscopy to detect abnormalities ultimately relies on the vigilance and experience of the operator. Like many screening tests, colonoscopy can result in a false negative (i.e., an abnormality may be overlooked by the operator), or a false positive (i.e., the operator may detect what he or she thinks is a malignancy or a suspicious lesion, but it may subsequently turn out to be totally harmless). The rate of false negatives (i.e., missed polyps or colorectal cancers) after colonoscopy is influenced by a number of factors.

For example, who performs the colonoscopy, and where it is carried out, can have a major influence on the reliability of the test. University of Western Ontario researchers have found that colonoscopy is far more likely to result in a false negative (i.e., cancer is more likely to be missed) when the test is performed by an internist or family physician, and when it is done in an office setting. Colonoscopy is far more accurate when done by a gastroenterologist in a hospital (Bressler 2007).

The senior author of this paper, Dr. Linda Rabeneck, has said: "There is something different about the practice of colonoscopy in these settings that gives rise to higher cancer miss rates, a worrisome finding" (Douglas 2007). Of 12,487 patients included in the study, 430 (or 3.4 percent) had new or missed cancer within 6 months to 3 years of having a colonoscopy. Reducing this interval to just 2 years yielded a 2.4 percent failure rate. An increase to 5 years gave a figure of 4.6 percent. Thus, to be really safe, patients may need to consider more frequent colonoscopies.

Compared to colonoscopy performed in the hospital, having the procedure done in a doctor's office yielded an odds ratio of new or missed colorectal cancer of 3.07 in men and 1.95 in women. In plain language, this means that you have a two to three times greater risk of a potentially fatal growth being missed just by having your colonoscopy done in an office, rather than a hospital! If you have it done by a family doctor it nearly doubles the missed cancer risk compared to gastroenterologist-performed colonoscopy.

Afternoon Delight?

The time of day you schedule your colonoscopy may also make a significant difference in the accuracy of the test. A recent study found that the colonoscopy failure rate was higher in the afternoon than when the procedure was performed in the morning (Sanaka 2006). The reason for this curious finding was partly that bowel preparation was not as thorough for afternoon colonoscopies, and partly because of the all-too-human factor of fatigue among endoscopists as the afternoon rolled around. Some doctors apparently weary of doing one after another of these somewhat tedious procedures. They tend to miss things when they get tired - as we all do. But in this case, lives are at stake.

This particular study involved 2,087 colonoscopies, roughly half of which were performed in the morning and half in the afternoon. The colonoscopy failure rate was 6.5 percent in the afternoon compared to just 4.1 percent in the morning, a significant difference of 2.1 percent. That could translate into quite a few lives saved - or lost - solely because of fatigue.

TO BE CONCLUDED, WITH REFERENCES, NEXT WEEK.

--Ralph W. Moss, Ph.D.