If you have original Medicare, then choosing which doctor you visit can make a big difference in how much you have to pay.

Under Medicare Part B, which pays for doctor visits, once your annual deductible is met, Medicare pays 80 percent of what it considers a “reasonable charge” for the item or service. You’re responsible for the other 20 percent.

However, in most cases, what Medicare calls a “reasonable charge” is less than what a medical provider normally charges for the service. Whether a Medicare beneficiary must pay part of the difference between the Medicare-approved charge and the provider’s normal charge depends on whether the provider has agreed to participate in the Medicare program.

If your doctor participates in Medicare, it means that the doctor “accepts assignment.” In other words, the doctor agrees that the total charge for the covered service will be the amount approved by Medicare. Medicare then pays the provider 80 percent of its approved amount, and the provider bills you for the remaining 20 percent.

But if your doctor doesn’t participate in Medicare and doesn’t accept assignment, the rules are different. Non-participating doctors can charge you 20 percent of the approved amount, plus up to an additional 15 percent more than the Medicare-approved amount.

Non-participating doctors can also charge you for the care upfront and request that you seek reimbursement from Medicare yourself, while doctors who accept assignment cannot do this.

As a result, it can make a big difference whether you choose a medical provider who participates in Medicare.

(Note that these rules apply to original Medicare, and not to seniors who are in Medicare Advantage, or managed care, plans.)