ACKGROUND The Centers for Medicare and Medicaid Services (CMS) administers the Medicare program. CMS employs Medicare contractors, including Wisconsin Physicians Service (WPS), to process and pay hospital outpatient claims using the Fiscal Intermediary Shared System (FISS). CMS implemented an outpatient prospective payment system (OPPS) for hospital outpatient services. Under the OPPS, Medicare pays for hospital outpatient services on a rate-per-service basis that varies according to the assigned ambulatory payment classification group. Under the OPPS, outlier payments are available when exceptionally costly services exceed established thresholds. Common medical devices implanted during outpatient procedures include cardiac devices, joint replacement devices, and infusion pumps. Generally, a provider implants only one cardiac device during an outpatient surgical procedure. Under the OPPS, payments to hospitals for medical devices are "packaged" into the payments for the procedures to insert devices. Hospitals are required to report the number of device units and related charges accurately on their claims. The failure to report device units and related charges accurately could result in incorrect outlier payments. Our audit covered $32,860 in Medicare outlier payments to hospitals for 14 claims for outpatient procedures that included the insertion of more than one of the same type of medical device. The 14