42 CFR §456.160

1. (a)Certification.
   1. (1)A physician must certify for each applicant or beneficiary that inpatient services in a mental hospital are or were needed.
   2. (2)The certification must be made at the time of admission or, if an individual applies for assistance while in a mental hospital, before the Medicaid agency authorizes payment.
2. (b)Recertification.
   1. (1)A physician, or physician assistant or nurse practitioner (as defined in § 491.2 of this chapter) acting within the scope of practice as defined by State law and under the supervision of a physician, must recertify for each applicant or beneficiary that inpatient services in a mental hospital are needed.
   2. (2)Recertification must be made at least every 60 days after certification.