42 CFR §409.80

1. (a)What they are.
   1. (1)The inpatient deductible and coinsurance amounts are portions of the cost of covered hospital or CAH or SNF services that Medicare does not pay.
   2. (2)The hospital or CAH or SNF may charge these amounts to the beneficiary or someone on his or her behalf.
2. (b)Changes in the inpatient deductible and coinsurance amounts.
   1. (1)The law requires the Secretary to adjust the inpatient hospital deductible each year to reflect changes in the average cost of hospital care. In adjusting the deductible, the Secretary must use a formula specified in section 1813(b)(2) of the Act. Under that formula, the inpatient hospital deductible is increased each year by about the same percentage as the increase in the average Medicare daily hospital costs. The result of the deductible increase is that the beneficiary continues to pay about the same proportion of the hospital bill.
   2. (2)Since the coinsurance amounts are, by statute, specific fractions of the deductible, they change when the deductible changes.