(AP) -- One in five medical claims is processed inaccurately by commercial health insurers, often leaving physicians shortchanged, according to the nation's largest doctor's group.

The American Medical Association released its third annual report card on insurers Monday.

In past years, Medicare performed well in how quickly and accurately it paid doctors, but the AMA did not release Medicare's data Monday to keep the focus on commercial insurers. Those matched their payments to what they agreed to pay doctors about 80 percent of the time.

The AMA has seen dramatic improvement from , said Dr. Nancy Nielsen, immediate past president of the group.

"It is the report card that forced them to pay attention," Nielsen said.

Insurers got a chance to see the report card in advance of Monday's release, she said.

The AMA rated Coventry Health Care Inc. highest of seven commercial insurers. Its national accuracy rating was about 88 percent. Anthem Blue Cross was at the bottom with an accuracy rating of 74 percent.

The AMA estimates that increasing the industry's accuracy to 100 percent would save doctors and insurers up to $15.5 billion a year.

The group is meeting in Chicago in its first annual meeting since the passage of President Barack Obama's health care overhaul.

The AMA report card is an effort to reduce the cost of claims processing for doctors. As much as $210 billion is spent annually just to process insurance claims.

Even though Medicare pays doctors accurately, the AMA is unhappy with the way the Medicare system sets payment rates for and has been lobbying for Congress to fix Medicare's reimbursement formula. AMA President James Rohack said Monday that both Democrats and Republicans "need to step up and fix this problem permanently."

Medicare said Monday it will hold doctors' claims through Thursday, giving lawmakers more time to prevent a 21 percent cut, required by a 1990s deficit reduction law Congress has routinely waived in the past. The fix, already approved by the House, is pending in the Senate. The cut was technically required as of June 1, but Medicare has been holding claims in hopes lawmakers will resolve the issue.