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The largest doctors group in the country is raising alarm that an obscure ObamaCare rule could stick them with the tab for patients who skip out on paying their premiums.

The American Medical Association, which originally supported the Affordable Care Act, warned the rule could pose a "significant financial risk" for doctors and hospitals, and on Wednesday blasted out guidelines to help members try and avoid those costs.

At issue is a 90-day "grace period" which lets patients who are not paying their premiums keep coverage for 90 days before it can be canceled.

Under the rule, insurers are responsible for paying any claims during the first month of that period -- but not necessarily for any claims during the final 60 days.

"Managing risk is typically a role for insurers, but the grace period rule transfers two-thirds of that risk from the insurers to physicians and health care providers," AMA President Ardis Dee Hoven said in a statement.

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The concern from physicians comes on top of widespread concerns from the insurance industry about the mix of new customers being signed up for coverage under the newly launched health insurance exchanges. The deadline for that coverage is March 31, and so far the Obama administration is lagging behind its enrollment projections.

But while insurance companies worry about having to take on costlier patients, medical practices are worried what happens when those patients stop paying their insurers.

The AMA has been urging the Obama administration to tweak the rules so that insurers are at least required to notify doctors as soon as a patient falls behind on insurance payments. In the interim, the guidelines sent out Wednesday were meant to help doctors and hospitals "minimize" those risks -- by, among other things, closely tracking grace-period notifications and checking whether state laws allow insurers to deny claims during these periods.

The main concern is that insurers could be allowed to place all claims incurred during the last 60 days in a "pending status" -- and then deny them if coverage ultimately is canceled.

But the administration argues that the grace-period rule is limited. It only applies to people who already have paid one month's premium, and requires insurers to tell doctors "as soon as practicable" when a customer is falling behind.

"Grace periods are important to ensure that consumers aren't dropped from coverage, especially for those experiencing economic challenges," said Alicia Hartinger, spokeswoman for the Centers for Medicare & Medicaid Services. "It is also important to us that providers know in a timely fashion whether their patients are in a grace period or not."

One possible -- and controversial -- solution for hospitals is for them to help struggling patients pay their premiums. But, as the AMA guidelines noted, the Department of Health and Human Services has strongly advised against this and threatened to take action if necessary.

The AMA said doctors "should exercise extreme caution" before even discussing that possibility with patients.