Understanding the Limitations of Cancer Registry Insurance Data

To the Editor Ellis et al report widening survival disparities for California patients with cancer who have public, primarily Medicaid, insurance relative to the privately insured. However, without an understanding of cancer registry data nuances and policy context, the study's implications for policy are easily misunderstood. The timing of insurance is not recorded in cancer registry data, and we cannot know whether insurance status changes during the course of treatment or whether patients were insured prior to their diagnosis (the insurance status reflected in the registry). The California Cancer Registry data presented by Ellis et al reflect the last known insurance status of multiple reports for a given tumor diagnosis, which may not accurately reflect insurance status immediately prior to diagnosis or even at first treatment. Many Medicaid beneficiaries gain coverage subsequent to diagnosis. Research from Michigan, a state that linked registry data with Medicaid enrollment records, found that more than 40% of patients with cancer who have Medicaid coverage enrolled after diagnosis. Ellis et al state that California Cancer Registry insurance data have been independently verified relative to other data sources, but it is unclear whether those sources contain information on the timing of insurance either (no reference is provided).

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