Using the National Death Index to Identify Duplicate Cancer Incident Cases in Florida and New York, 1996–2005

INTRODUCTION: Cancer registries link incidence data to state death certificates to update vital status and identify missing cases; they also link these data to the National Death Index (NDI) to update vital status among patients who leave the state after their diagnosis. This study explored the use of information from NDI linkages to identify potential duplicate cancer cases registered in both Florida and New York. METHODS: The Florida Cancer Data System (FCDS) and the New York State Cancer Registry (NYSCR) linked incidence data with state and NDI death records from 1996 through 2005. Information for patients whose death occurred in the reciprocal state (the death state) was exchanged. Potential duplicate cases were those that had the same diagnosis and the same or similar diagnosis date. RESULTS: NDI identified 4,657 FCDS cancer patients who died in New York and 2,740 NYSCR cancer patients who died in Florida. Matching identified 5,030 cases registered in both states; 508 were death certificate-only (DCO) cases in the death state’s registry, and 3,760 (74.8%) were potential duplicates. Among FCDS and NYSCR patients who died and were registered in the registry of the reciprocal state, more than 50% were registered with the same cancer diagnosis, and approximately 80% had similar diagnosis dates (within 1 year). CONCLUSION: NDI identified DCO cases in the death state’s cancer registry and a large proportion of potential duplicate cases. Standards are needed for assigning primary residence when multiple registries report the same case. The registry initiating the NDI linkage should consider sharing relevant information with death state registries so that these registries can remove erroneous DCO cases from their databases.