42 CFR § 424.506 - National Provider Identifier (NPI) on all enrollment applications and claims.
(a) Definition. Eligible professional means any of the professionals specified in section 1848(k)(3)(B) of the Act.
(b) Enrollment requirements.
(1) A provider or a supplier that is eligible for an NPI must do the following:
(ii) If the provider or supplier was in the Medicare program before obtaining an NPI and the provider's or the supplier's NPI is not in the provider's or supplier's Medicare enrollment record, the provider or supplier must update its Medicare enrollment record by submitting its NPI using either of the following:
(A) The applicable paper CMS–855 form.
(B) Internet-based PECOS.
(2) A physician or eligible professional who has validly opted-out of the Medicare program is not required to submit a Medicare enrollment application for any reason, including to order or certify.
(c) Claims reporting requirements.
(i) Must include the legal name of any provider or supplier who is required to be identified in that claim; and
(3) A Medicare contractor will reject a claim from a provider or a supplier if the required NPI(s) is not reported.