People who've had a colonoscopy with no signs of cancer may be able to delay the next one or not have it at all, German researchers suggest.

Guidelines from the U.S. Preventive Services Task Force, a federally supported expert panel, currently recommend that people at average risk of colon cancer start screening for the disease at age 50, using one of a number of tests that have all been found to cut the risk of dying from colon cancer.

The advantage of colonoscopy, which costs around $3,000, is that it only has to be repeated once every ten years, as opposed to every year for the much cheaper stool test.

Although the German study isn't definitive, it does suggest that doing even fewer colonoscopies might be reasonable in some cases.

Dr. Hermann Brenner at the German Cancer Research Center in Heidelberg and colleagues asked nearly 2,000 colon cancer patients and 2,400 healthy people whether and when they'd had colonoscopies.

Compared with people who'd never undergone the procedure, those whose test came back negative—meaning they had no trace of colon tumors—had five times lower odds of developing cancer down the road.

Writing in the Journal of Clinical Oncology, the German researchers say their results challenge current guidelines.

"The finding of sustained low risk for 20 years and beyond after a negative colonoscopy suggests that a screening colonoscopy might not need to be repeated after 10 years as was previously recommended," they note.

For those who light up, however, that might not be the case. Ten years from a negative test, current smokers had the same colon cancer risk as non-smokers who'd never had a colonoscopy.

According to the American Cancer Society, about one in 20 people get colon cancer, and the disease causes nearly 50,000 U.S. deaths every year.

Longer screening intervals could have a significant impact on the overall cost-effectiveness of colonoscopy as well as lower the rate of colonoscopy-related complications, according to the German team.

Dr. John Inadomi of the University of Washington in Seattle said the new findings are not surprising, because a negative colonoscopy sorts people at low risk of colon cancer from those at higher risk.

He added that the first intervention to prevent a disease is always the most effective because of diminishing returns on any subsequent interventions that cost the same.

Still, he told Reuters Health, "perhaps lengthening the interval between screening colonoscopies is a reasonable proposal" for patients with a negative colonoscopy.