42 CFR § 431.52 - Payments for services furnished out of State.
(b)Payment for services. A State plan must provide that the State will pay for services furnished in another State to the same extent that it would pay for services furnished within its boundaries if the services are furnished to a beneficiary who is a resident of the State, and any of the following conditions is met:
(3) The State determines, on the basis of medical advice, that the needed medical services, or necessary supplementary resources, are more readily available in the other State;
(4) It is general practice for beneficiaries in a particular locality to use medical resources in another State.
(c)Cooperation among States. The plan must provide that the State will establish procedures to facilitate the furnishing of medical services to individuals who are present in the State and are eligible for Medicaid under another State's plan.