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In Original Medicare, according to an example provided by Kaiser, a $100 doctor’s bill involving a participating provider would cost you a 20 percent copayment, or $20, after Medicare’s 80 percent payment under Part B insurance rules. If you were, instead, seeing a nonparticipating provider, Medicare would first reduce its allowable fee for the service by 5 percent, from $100 to $95. If your doctor agreed to assignment here, Medicare would cover $76 of the $95 bill (its 80 percent payment) and you would pay the remaining $19. But if the doctor did not accept assignment, he or she would be able to charge you up to 115 percent of the reduced charge. In our example, this would total up to $109.25 (115 percent of $95). Medicare would still pay $76. But now you would pay not $19 but as much as $33.25. In the real world, of course, especially if the doctor involved was performing surgery, you would be multiplying this $100 many, many times over. And balance billing could definitely unbalance your budget.
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Philip Moeller (Get What's Yours for Medicare: Maximize Your Coverage, Minimize Your Costs (The Get What's Yours Series))