What makes a good colonoscopy? A recent study published in The New England Journal of Medicine reveals that not all doctors use the same degree of care when they look for adenomatous polyps (those that have a risk of developing into colon cancer). In fact, some doctors were four times better than others at finding these adenomas.

Researchers concluded that success at detection of polyps depends on the amount of time doctors spend on the most crucial part of a colonoscopy -- the slow removal of the endoscope, when most abnormalities are spotted and where there is the most variation in the examiners' speed of removal.

Experts recommend spending at least six to 10 minutes for this part of the colonoscopy. Slower withdrawal times yield greater detection rates. But in this study, some of the doctors were not slow at all. The time spent ranged from three to 17 minutes for procedures in which no polyps were removed. Those gastroenterologists who spent more time -- a minimum of six minutes -- had more than double the detection rate of those who took less time: 28% vs. 12% for detecting any polyps and 6% vs. 3% for finding advanced adenomatous polyps.

Because the study looked at only 12 gastroenterologists in private practice, the authors were careful to say the results are preliminary. However, the researchers did review 2,053 colonoscopies performed on average-risk patients. What's more, this is not the first study to question the skill of colonoscopy examiners: Earlier studies have found that during colonoscopy some doctors miss as many as 15–27% of polyps that have reached the adenoma stage. Still another study compared the regular colonoscopy with the so-called virtual colonoscopy, which images the interior of the colon using computed tomography (CT). Both procedures were done on the same day on the same patient. Both techniques missed some polyps.

This information should not discourage you from having a colonoscopy. Colonoscopy is still the best way to check for colon cancer: The American Cancer Society says colorectal cancer deaths dropped 25% between 1990 and 2003, partly because more people are being screened and thus treated earlier. And doctors want to do their best. When the 12-doctor study was published, the gastroenterologists involved began using timers to ensure longer examinations. Their detection rate has doubled.

Evidence suggests that the best place to get your colonoscopy is a hospital and the best healthcare professional to perform the colonoscopy is a gastroenterologist. A Canadian study of more than 12,000 patients found that men and women who had their colonoscopies performed in an office by a family physician or an internist were more likely to have a new or missed colorectal cancer than people who had the procedure performed in the hospital by a gastroenterologist.

What Can You Do To Help? Here is advice from gastroenterologists on getting an effective colonoscopy.

Ask questions before the colonoscopy. You will have a mild sedative for the colonoscopy, so be prepared to discuss your concerns before you receive the medication -- or even better, before the day of the colonoscopy.

Do your part in the prep. If you haven't cleaned your colon thoroughly, the doctor will not have a clear view, and having the most skilled and careful examiner will not make much difference.