Incident to?

You need to be more specific in your query. There are numerous issues with this and it is one of the highest priority issues with the OIG. SO make sure you follow everything to the letter. Medicare defines it as
* the physician must have already seen the patient for this same issue
* there must be a plan of care in the physician notes detailing the need for the return encounter and what is to be done
* The provider you are billing under must be on site; within the narrow confines of the office suite area

If this is a new patient or a new problem or a consultation, or the provider is not on site you cannot bill "incident to".
Most if not all commercial carriers follow this definition 100%. If you have a carrier that expresses they do not credential the mid level providers and instruct you bill under the physician , you must obtain this policy in writing! The policy must acknowledge that the carrier understands the provider may have never treated this patient and may not be on site at the time of service.