I think this is one of those gray areas and may be one that is determined by the actual carrier. If we followed CPT's instructions...
When one group provides coverage for another physician group, the patient encounter is classified as it would have been by the physician who is not available. For example, let's say your practice provides coverage for a solo physician in your community. While the physician is out of town, you see one of her patients.
As long as the physician who is out of town has seen the patient in the last three years, you have to report the service using an established patient code. This is true even if you are unfamiliar with the patient, clinical information is not available and the office staff does not have basic demographic information.
However...thinking from a CMS perspective, I don't think the scenario would produce the same result.
Special considerations for Medicare patients
A slightly different approach may be taken when Medicare patients are involved. Medicare has stated that a patient is a new patient if no face-to-face service was reported in the last three years. The group practice and specialty distinctions still apply, but "professional service" is limited to face-to-face encounters. Therefore, if you see a Medicare patient
whom you have seen within the last three years, you
must report the service using an established patient code. On the other hand, if a lab interpretation is billed but no face-to-face encounter took place, the new patient designation might be appropriate.
I think this is one of those areas where you have to consider both guidelines and determine which one the carrier in question follows.
http://www.aafp.org/fpm/20030900/33unde.html