42 CFR §417.528

1. (a)Limits on payments and charges.
   1. (1)CMS may not pay for services to the extent that Medicare is not the primary payer under section 1862(b) of the Act and part 411 of this chapter.
   2. (2)The circumstances under which an HMO or CMP may charge, or authorize a provider to charge, for covered Medicare services for which Medicare is not the primary payer are stated in paragraphs (b) and (c) of this section.
2. (b)Charge to other insurers or the enrollee. If a Medicare enrollee receives from an HMO or CMP covered services that are also covered under State or Federal worker's compensation, automobile medical, or any no-fault insurance, or any liability insurance policy or plan, including a self-insured plan, the HMO or CMP may charge, or authorize a provider that furnished the service to charge—
   1. (1)The insurance carrier, employer, or other entity that is liable to pay for these services; or
   2. (2)The Medicare enrollee, to the extent that he or she has been paid by the carrier, employer, or other entity.
3. (c)Charge to group health plans (GHPs) or large group health plans (LGHPs). An HMO or CMP may charge a GHP or LGHP for covered services it furnished to a Medicare enrollee and may charge the Medicare enrollee to the extent that he or she has been paid by the GHP or LGHP for these covered services if—
   1. (1)The Medicare enrollee is covered under the plan; and
   2. (2)Under section 1862(b) of the Act, CMS is precluded from paying for the covered services .
4. (d)Responsibilities of HMO or CMP. An HMO or CMP must—
   1. (1)Identify payers that are primary to Medicare under section 1862(b) of the Act;
   2. (2)Determine the amounts payable by these payers; and
   3. (3)Coordinate the benefits of its Medicare enrollees with these payers.