who is responsible for Medicare 20% coinsurance?

Part B Coinsurance

After the deductible has been satisfied, coinsurance of 20 percent is usually applicable.

For providers and suppliers that bill intermediaries, the 20 percent may be based on the allowed amount, billed charges, or a preset rate per service (APC), depending upon the type of service. See Claims Processing instructions for a description of coinsurance calculation for each benefit type.

Physicians and other suppliers will be paid 80 percent of allowed amount under the fee schedule amounts or in some instances reasonable charges incurred during the balance of the calendar year. The patient is responsible for a coinsurance amount equal to 20 percent of the fee schedule amounts or reasonable charges for the items and services.