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Many patients have a repeat colonoscopy sooner than the recommended 10-year wait.

Many older Americans get repeat colon cancer tests they don't need and Medicare is paying for it, suggests a study that spotlights unnecessary risks to the elderly and a waste of money.

Almost half of the Medicare patients in the study had had a colonoscopy less than seven years after getting normal results from an earlier test. The test is recommended just every 10 years, starting at age 50, for people at average risk whose initial test is normal.

The study showed that among those 80 and older, one-third had a repeat exam within seven years of the previous colonoscopy. That's an age group that can skip the test altogether if no problems have been spotted before.

The U.S. Preventive Services Task Force recommends against routine colon cancer screening for most people 76 to 85. It says for those older than 85, screening risks outweigh the benefits, since the older you are, the more likely you are to die from other causes before cancer becomes deadly.

"I was surprised by the magnitude of the issue," said lead author Dr. James Goodwin, a geriatrician and researcher at University of Texas Medical Branch in Galveston.

In the study, researchers chose a random national sample of Medicare claims and enrollment data from more than 200,000 patients over 65 who received colonoscopies between 2001 and 2008. The number of patients in the sample totaled 24,071 - all people considered at normal risk for colon cancer.

Only 27 percent of all study patients with frequent exams had symptoms that might have raised suspicion of cancer, including abdominal pain, change in bowel habits, and weight loss. The study appears in Archives of Internal Medicine.

The colonoscopy, where a doctor uses a thin flexible tube to examine the intestines, is considered one of the most effective screening tests available, and it's credited with saving thousands of lives by catching cancer early.

The exam is generally pretty safe, but does have risks that occur more often with older patients, including complications from sedation, accidental perforation of the colon and bleeding.

Medicare covers colonoscopies every 10 years - more frequently for high-risk patients. While Medicare rules say the government won't pay for too-frequent colonoscopies, only 2 percent of the study claims were denied for repeat exams in people without symptoms. Colonoscopy costs typically exceed $1,000.

Robert Smith, director of cancer screening at the American Cancer Society, said some doctors may recommend more frequent colonoscopies because they think 10-year intervals are too risky. Some may think, incorrectly, that finding any growths, even nonsuspicious polyps, means a repeat exam should be done in less than 10 years, Smith said.

Some doctors also order repeat tests "because they want to bring in income," he said, noting that in contrast, the uninsured, blacks and Hispanics don't get enough colon cancer screening.

If the study results are true, "then we need to further validate the accuracy of our payments," said Ellen Griffith, spokeswoman for the U.S. Centers for Medicare & Medicaid Services.