Hello,
If the patient hasn't been seen by the provider personally, or by another provider with the same specialty and subspecialty within 3 years, then the patient is considered a New Patient. If the reverse is true, then the patient is an Established Patient for billing and coding purposes.
For example:
New provider
A is a cardiologist and starts seeing patient. If another provider
B from the same practice who is also a cardiologist and has seen provider
A's patient before and within 3 years, then -> Established (same specialty).
If the provider is a Nurse Practitioner, then there are some special rules on credentialing, but I'm gathering from your post that the new provider is a physician.
Some references:
AMA CPT 2018 book, New and Established Patient, page 1:
“A new patient is one who has not received any professional services from the physician/qualified health care professional or another physician/qualified health care professional of the exact same specialty and subspecialty who belongs to the same group practice, within the past three years”.
Noridian (MAC)
Hope that helps!