There are two types of supervision required under Medicare regulations. One applies to "incident to" services and the other to "physician services."
"Incident to" services are those which are integral, although incidental, to a physician's professional service that is commonly furnished in a physician's office. It requires that the physician "be present in the office suite and immediately available to provide assistance and direction throughout the time the physician assistant is performing services." However, this does not require the physician to be physically present in the same room as the physician assistant only in the office suite.
Physician services can also be performed by physician assistants. These are broadly defined under Medicare as "the type that are considered physician's services if furnished by a doctor of medicine or osteopathy." Again, the scope of these services would be limited by state law. The physician must be immediately available for consultation purposes by telephone or "other effective, reliable means of communication."
If a patient presents with a new problem that is within the Physician Assistant's scope of practice per state or organization he/she may evaluate the new problem and billing must be under the PA as a direct bill. In other words...not under the physician's name and number. If the physician enters the scene shared billing may be appropriate if allowed by the payer. Remember incident - to is only for Medicare.
Hope this is helpful.