40 PA.C.S §3803(a-b)

(a) General rule.–Except as provided in section 3804 (relating to exceptions to retroactive denial of reimbursement), an insurer may not retroactively deny reimbursement as a result of an overpayment determination more than 24 months after the date the insurer initially paid the health care provider. An insurer that retroactively denies reimbursement to a health care provider under this chapter shall do so based upon coding guidelines and policies in effect at the time the service subject to the retroactive denial was rendered.

(b) Written notice.–An insurer that retroactively denies reimbursement to a health care provider under subsection (a) shall provide the health care provider with a written statement specifying the basis for the retroactive denial.