(a) For the purpose of determining underpayment adjustments to a Medi-Cal recovery action, underpayment shall mean a payment amount within the audit period identified by the pharmacy or clinic with a special permit as being deficient. The underpayment may be due to a billing error made by the pharmacy or clinic with a special permit or an incorrect payment made by the fiscal intermediary for claims submitted within the billing time limits as specified in Section 51008.

(b) The pharmacy or clinic with a special permit shall submit a written statement of findings of underpayment to the Department of Health Services prior to issuance of a demand for payment of identified underpayments. This written statement shall provide specific documentation pursuant to Section 51476 to support the contention of the pharmacy or clinic with a special permit that an underpayment occurred.

(c) Should a probability sample be used in auditing a pharmacy or a clinic with a special permit having qualified underpaid amounts in the sample, both the overpayments and underpayments shall be extrapolated to the audit period universe in determining the recoverable amount due.

(d) No review may be reopened to provide for underpayments in which a final decision has been reached pursuant to Section 14171 of the Welfare and Institutions Code.

(e) For the purpose of this section, a valid claim shall mean any claim, for service dates within the audit period, that meets all of the requirements for billing established under Articles 1, 6, and 7 of this Chapter.