That leaves several other possibilities.

Perhaps patients did not sufficiently cleanse their bowels of fecal material, a particular problem for the right side of the colon.

“After the prep has been completed, mucus and intestinal secretions start rolling out of the small intestine and colon,” Dr. Rex explained. The secretions, he added, pour from the base of the appendix into the right side of the colon and are “very sticky” and can obscure polyps.

One solution, supported by six studies, is to be sure there is just a short time between when patients finish taking the strong laxative that cleanses their bowel and the colonoscopy, Dr. Rex said. That usually means taking half of the laxative the night before the screening test and the rest in the morning, something that often is not done, he added, but that he and others recommend.

Cancer may also be different in the right colon, researchers said.

Flat and indented polyps tend to cluster in the right colon. And so do another kind, serrated lesions, which, some studies indicate, might turn into cancer much more quickly than typical polyps.

“It’s possible that we will never get as good a result” in the right colon, said Robert Smith, director of screening for the American Cancer Society.

Still, he said, that does not mean that patients should have more frequent colonoscopies. The tests are “hugely expensive,” he said, and insurers may not pay for more frequent colonoscopies. The test also carries a small risk of perforating the bowel. Even if colonoscopies miss some cancers, colon cancer remains a rare disease and, after a colonoscopy, “the likelihood that you have cancer is very, very low,” Dr. Smith said.

Dr. Harold C. Sox, editor of the Annals of Internal Medicine, is choosing another option. He is having a stool test, the fecal occult blood test, between colonoscopies. It looks for blood in the stool, which can arise from colon cancer.