The decision seems to be determined by the supervising physician and his/her scope of practice.
http://www.wapa.org/Professional-Practice/Rules-and-Regulations
http://www.wapa.org/docs/Physician-Assistant-Scope-of-Practice
http://www.wapa.org/docs/Top-Ten-PA-Practice-Questions
A physician assistant's practice may not exceed his or her educational training or experience and may not exceed the scope of practice of the supervising physician.
As our scope of responsibilities is determined by the practice of our supervising physician, we function as a medical team member who provides a broad range of services. These services may include:
patient histories and physical exams
a variety of diagnostic studies to form a diagnostic impression
initiation and management of therapies for acute or chronic health problems
health screenings, preventive care, patient education, and counseling
minor surgical procedures
family planning, perinatal, and gynecological care
assisting with surgery, ER, acute hospital, and long-term care
referral and follow-up care with physician specialists
issuing prescription orders for medications