Section 20-2-IV - PHYSICAL THERAPIST ASSISTANT OR OCCUPATIONAL THERAPIST ASSISTANTA. Mississippi-specific modifier M3 should be appended to the appropriate CPT code(s) when billing services rendered by a physical therapist assistant or an occupational therapist assistant.B. The physical therapist assistant or occupational therapist assistant is reimbursed at eighty-five percent (85%) of the maximum allowable for the procedure.