The code (99213 or 99214) doesn't really matter but the modifier for "incident to" matters a lot.
From the ACNP website:
What is "incident to billing"?
Q. What is "incident to billing"?
A. "Incident to" billing is a Medicare term which can be applied to patients being billed under the fee for service system with Medicare. Medicare's "incident to" billing has specific requirements which must be met to bill physician services "incident to" a physician.
Medicare Regulations at 42 C.F.R. Section 410.26 provide that Medicare Part B will pay for services and supplies incident to a physician's professional services, including drugs and biologicals that cannot be self-administered, as long as the services or supplies are of the type that are commonly furnished in a physician's office or clinic, and are commonly furnished either without charge or are included in the physician's bill.
The Medicare Carrier's Manual list the criteria are for a service to qualify as "incident to" the services of a physician. To quality for reimbursement, the service must be an integral part of the physician's professional service, commonly provided without charge or included in the physician's bill; commonly furnished in physician offices; and furnished by the physician or by an individual who qualifies as an employee of the physician.
A physician can bill for the services of a nurse practitioner if the nurse practitioner's services are billed incident to the physician's services; however to do so, the physician must meet certain supervision requirements. The services must be performed under the direct personal supervision of the physician as an integral part of the physician's personal in-office service. Such direct personal supervision requires that the physician initiate the course of treatment for which the service being performed by the nurse practitioner is an incidental part and that the physician remain actively involved with the patient's care. The physician must also be physically present in the same office suite and be immediately available to render assistance if necessary. In addition, the nurse practitioner must be employed by the physician (or be a leased employee). Services provided by auxiliary personnel not in the employ of the physician, even if included in the physician's bill, are not covered as incident to a physician's service.
The advantage of billing "incident to" is the practice receives 100% of the physician fee schedule for the service. The disadvantage is all of the incident to requirements must be followed. If a nurse practitioner bills directly for his or her services under their own Medicare provider ID number, the nurse practitioner receives 85% of the physician fee schedule. The advantage is that the restrictive "incident to" rules do not apply for nurse practitioners billing directly under their own number. Incident to billing is not permitted in hospitals, skilled nursing facilities or in the home.