Two studies of colonoscopy screening reported in the May 24 and 31 Journal of the American Medical Association (JAMA) fortified evidence of the test's value, and "address two very important and unresolved questions," according to an accompanying editorial.

Is 10 years the right amount of time to wait for a second colonoscopy after a negative initial colonoscopy exam? Absolutely, according to researchers at the University of Manitoba in Winnipeg, Canada, who conducted a retrospective population-based analysis using a cohort of 35,975 patients screened over a period of 15 years from 1989 to 2003.

The second question is whether people 80 years of age and older will increase their life expectancy by colonoscopy screening.

The answer here, according to researchers who followed 1,244 individuals after colonoscopy screening at the Virginia Mason Medical Center in Seattle, is yes, but not by much: Those over age 79 gained only 0.13 years by screening, which is just 15 percent of the 0.85 years gained by those aged 50 to 54.

"Very elderly patients and their physicians are using individual judgment to decide whether to undergo screening" with colonoscopy, explains lead author Dr. Otto S. Lin. While studies show that high rates of colorectal neoplasia are likely to be detected, he adds, the slow rate at which these cancers progress means they are much less likely to be the cause of death in the elderly. And those considerations must be weighed against the risks associated with screening, including bowel perforation and gastrointestinal bleeding.

The Canadian study, led by Dr. Harminder Singh, examined how risk changes over time in those whose index colonoscopy detected no polyps. Researchers compared the incidence of colorectal cancer among the cohort with that of the general population of the province of Manitoba. Colonoscopy screening reduced cohort members' risk by 31 percent 6 months after the initial exam.

Over the next 10 years, their risk of colorectal cancer actually decreased because any cancers that were missed by the initial screening presented themselves sooner rather than later, thus leaving truly cancer-free people in the cohort. At 1 year, risk is down to 66 percent compared with the general population; at 2 years, 59 percent; at 5 years, 55 percent; and at 10 years, risk falls to 28 percent.

These results reinforce the current recommendation by the American College of Gastroenterology (ACG), which is based on observational studies: By age 50, individuals should undergo a colonoscopy every 10 years. Since the cost of screening 1 in 10 Americans over age 50 each year is more than $4 billion, reasoned Dr. Timothy R. Church of the University of Minnesota in the editorial, "it may well be time" for a randomized controlled trial to definitively assess the efficacy of colonoscopy.

Although colonoscopy is preferred by ACG, the U.S. Preventive Services Task Force, the Centers for Disease Control and Prevention, and the American Cancer Society, each recommends slightly different guidelines for different populations, including at least four other tests that are routinely available: the fecal occult blood test (FOBT), flexible sigmoidoscopy, digital rectal exam, and double contrast barium enema.

What is clear, said Dr. Asad Umar, acting chief of the Gastrointestinal and Other Cancers Research Group in NCI's Division of Cancer Prevention, "is that the majority of Americans over 50 are not getting screened every 10 years via colonoscopy. Following alternative recommendations, everyone should be screened by FOBT annually, or by flexible sigmoidoscopy or double contrast barium enema every 5 years because three out of four colorectal cancers strike people with no family history or other known risk factors, and are very treatable when detected early."

While colorectal cancer is expected to kill 55,170 Americans in 2006, it's estimated that 33,000 of these deaths could be prevented if people aged 50 years or older underwent regular screenings.

By Addison Greenwood