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Feasibility of Matching Medicare and Medicaid Data for Dually Eligible Beneficiaries in Oregon

Publication Date
Feb 27, 1999

U.S. Department of Health and Human Services

Feasibility of Matching Medicare and Medicaid Data for Dually Eligible Beneficiaries in Oregon

Executive Summary

Carol J. Ammering, B.S., and Debra A. Dayhoff, Ph.D.

Health Economics Research, Inc.

September 28, 1999

This report was prepared under contract #500-94-0056 between the U.S. Department of Health and Human Services (HHS), Office of Disability, Aging and Long-Term Care Policy (DALTCP) and Health Economics Research, Inc. Additional funding was provided by the Health Care Financing Administration. For additional information about this subject, you can visit the DALTCP home page at or contact the ASPE Project Officer, Hunter McKay, at HHS/ASPE/DALTCP, Room 424E, H.H. Humphrey Building, 200 Independence Avenue, S.W., Washington, D.C. 20201. His e-mail address is:

The opinions and views expressed in this report are those of the authors. They do not necessarily reflect the views of the Department of Health and Human Services, the contractor or any other funding organization.

In 1994, the Oregon Health Plan (OHP) was started under an 1115 waiver demonstration. As part of this waiver, beneficiaries were moved from fee-for-service to managed care, eligibility was expanded to all persons below the Federal Poverty Level, and a prioritized list of services replaced the standard Medicaid benefit package. Unlike most states, Oregon received approval from the Health Care Financing Administration (HCFA) to move those beneficiaries eligible for both Medicare and Medicaid (dual eligibles) into Medicaid managed care. One year after the implementation of OHP, the transition of dual eligibles into managed care was started.

This study is unique in that it attempts to link Medicare claims with Medicaid managed care encounters and with Medicaid fee-for-service claims. Previous researchers have merged Medicare and Medicaid claims files for dually eligible beneficiaries in fee-for-service, but such a task has not been previously attempted for those in managed care. Under managed care, traditional claims are not submitted, but plans are required by the State to submit encounter data. In this study, we assess the feasibility of matching 1996 Medicaid encounter data to the corresponding Medicare claim. By providing a claim (or encounter) to claim match, a complete picture of the cost and utilization of services by the dual eligible population can be portrayed.

We were able to link beneficiary IDs across Medicare and Medicaid eligibility files and to determine periods of eligibility for coverage. We also developed algorithms to match corresponding Medicare and Medicaid claims/encounters, using a variety of criteria. We first attempted to match Medicare claims to Medicaid claims/encounters using date of service, procedure and diagnosis. This resulted in a match rate of about 12 percent. Even after relaxing the criteria constituting a "match" to the date of service on the Medicare and Medicaid files being within two days of each other, only a relatively small percentage of Medicare claims had a "matching" Medicaid claim or encounter:

  • 29 percent of Medicare physician claims had a matching Medicaid claim or encounter;
  • 10 percent of Medicare outpatient department services linked to a Medicaid claim/encounter; and
  • 18 percent of Medicare inpatient stays had a Medicaid counterpart.

Mental health claims had the highest rate of matching (54%) among procedure groupings for the physician claims. We suspect that the higher rate results from the higher Medicare co-payment rate for mental health services (50% vs. 20% for other covered services). This supports the theory that Medicaid cost-sharing claims/ encounters are not present in the data because providers do not bother to submit them, given the low payment levels provided by Medicaid. If the Medicare payment (less the deductible and copayments) is higher than the State Medicaid reimbursement rate, the State will not pay the cost-sharing. Thus, a provider will not receive Medicaid cost-sharing if the provider would then get more than they would have been paid by Medicaid for the service.

Since the absolute numbers of Medicare claims and Medicaid claims/encounters for dually eligible beneficiaries were similar, we were surprised by the low match rates. To investigate further, we reversed the process and matched Medicaid data to Medicare claims. Only 38 percent of Medicaid encounters and 24 percent of Medicaid fee-for-service claims matched a Medicare claim. There are two explanations for the low match rates. First, Medicaid claims/encounters would have no matching Medicare claim if the beneficiary was enrolled in Medicare managed care. Second, the majority of the services generating a Medicaid claim/encounter are for non-Medicare covered services (e.g., transportation, vision, school-based services, non-covered mental health services).1

Despite the low matching rates, Medicare claims and Medicaid claims and encounters can still provide a detailed representation of cost and utilization for dually eligible beneficiaries. Medicaid claims and encounters serve to provide data on services not provided by Medicare. However, because Medicare HMOs do not provide encounter data, information for beneficiaries enrolled in Medicare HMOs will remain incomplete.


  1. Non-Medicare covered mental health services found in the Medicaid claims were daily structure and support services and skill training services.

The Full Report is also available from the DALTCP website ( or directly at
Medicare Beneficiaries
Location- & Geography-Based Data
State Data