In addition to their monthly premium (see Who's eligible for Medicare Part B coverage--and who pays? above), participants are likely to be responsible for a large portion of their doctor and other outpatient medical bills, despite Part B coverage. Why is this? First, there's a yearly deductible ($135) each participant has to pay before Medicare will pay anything. Also, when Part B pays only part (usually 80 percent) of an approved charge, participants are personally responsible for the unpaid remainder.

Participants may also be responsible for the difference between the Medicare-approved amount and what the doctor or other provider actually charges. By law, this can't exceed 15 percent more than the approved charge.

Whether someone is charged this extra 15 percent depends on whether the doctor or other provider accepts "assignment." This means the provider accepts the Medicare-approved charge as the entire amount of the bill. Most doctors accept assignment of Medicare charges -- but not all will do so, and many outpatient clinics do not. So participants need to check on their doctor's or provider's policy about assignment before beginning any care. To fill the unpaid gaps in Part B coverage and payment, participants may want to buy a Medigap insurance policy. Or, if they have low income and few assets, they can look to Medicaid or other programs for low-income seniors for help paying these bills.