THURSDAY, March 29 (HealthDay News) -- Doctors may fail to detect precancerous growths if patients' colons aren't adequately cleansed before having a colonoscopy, a new study warns.

On the day before a colonoscopy, patients are instructed to stop eating solid food and to consume only clear liquids. They also drink bowel-cleansing mixtures to empty the colon.

However, previous research suggests that up to one-quarter of colonoscopy patients fail to adequately cleanse their colons. The new study, from the Washington University School of Medicine in St. Louis, examined the potential consequences of poor bowel preparation.

Researchers identified 373 patients who underwent a colonoscopy between 2004 and 2009, and had inadequate bowel preparation. Of the 133 patients who later had a second colonoscopy, nearly 34 percent had at least one precancerous growth detected in the repeat screening.

Nearly one in five of those patients was considered to be at high risk for colon cancer because they either had more than three precancerous growths detected, or at least one large growth.

The researchers also found that 18 percent of the patients who had a second colonoscopy would have received different recommendations from doctors if their precancerous growths had been detected during the initial colonoscopy. For example, they may have been advised to have more frequent colonoscopies to monitor the growths in their colon.

The study was released online Feb. 28 in advance of publication in an upcoming print issue of the journal Gastrointestinal Endoscopy.

"Because so many of the patients had a follow-up screening less than a year after the initial test, we strongly suspect that most of the precancerous growths found during the second colonoscopy already were present at the time of the initial test," first author and gastroenterology fellow Dr. Reena Chokshi said in a university news release.

The findings suggest that if a doctor is having difficulty viewing a patient's colon because of inadequate bowel prep, the colonoscopy should be stopped and rescheduled.

"We often can detect preparation problems during the first few minutes of the procedure," Chokshi said. "And based on this study, we would say that rather than subjecting a patient to the potential risks of a full colonoscopy when we may not be able to detect polyps, or other precancerous growths called adenomas, it may be better to bring that patient back as soon as possible for a repeat procedure with better bowel preparation."

More information

The U.S. National Institute of Diabetes and Digestive and Kidney Diseases has more about colonoscopy.