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A prominent U.S. senator is asking the Centers for Medicare & Medicaid Services to investigate Medicare Advantage plans that may have inflated their risk scores to receive higher reimbursements.

In her letter to CMS Acting Administrator Andy Slavitt, Sen. Claire McCaskill (D-MO) expressed concern that the practice has been going on for years.

“There is evidence that Medicare Advantage plans have been inflating risk scores in order to reap more money from the Medicare Trust Funds,” McCaskill wrote, noting that at least six whistleblower lawsuits alleging such practices have been filed since 2010. “I find this extremely troubling as over 15 million Americans, about 30% of the total Medicare population, are enrolled in MA plans and in fiscal year 2014 alone, the Government Accountability Office [GAO] reported more than $12 billion in improper payments were made to Medicare Advantage Plans.”

In a prepared statement published Thursday, McCaskill added, “Gaming the Medicare Advantage system is a threat to taxpayers, but also to Medicare beneficiaries — and we should be doing everything possible to aggressively pursue anyone engaged in wrongdoing.”

Medicare Advantage plans are rewarded with higher payments for higher risk scores because they reflect better beneficiary health status.

A CMS spokesperson told reporters the agency is currently conducting risk-adjustment data validation audits to verify the accuracy of MA sponsors’ reporting.