42 CFR §409.26

1. (a)Services furnished by an intern or a resident-in-training. Medicare pays for medical services that are furnished by an intern or a resident-in-training (under a hospital teaching program approved in accordance with the provisions of § 409.15) as posthospital SNF care, if the intern or resident is in—
   1. (1)A participating hospital with which the SNF has in effect an agreement under § 483.70(j) of this chapter for the transfer of patients and exchange of medical records; or
   2. (2)A hospital that has a swing-bed approval, and is furnishing services to an SNF-level inpatient of that hospital.
2. (b)Other diagnostic or therapeutic services. Medicare pays for other diagnostic or therapeutic services as posthospital SNF care if they are provided—
   1. (1)By a participating hospital with which the SNF has in effect a transfer agreement as described in paragraph (a)(1) of this section; or
   2. (2)By a hospital or a CAH that has a swing-bed approval, to its own SNF-level inpatient.