Many providers rely on electronic billing systems to report information to immunization registries. If billing data fail to capture some administered immunizations, the registry will not reflect a child's true immunization status. Our objective was to assess differences between immunizations administered and immunizations reported to a registry from electronic billing systems. Philadelphia's Department of Public Health conducted chart audits in 45 providers serving 50 or more children aged 7-35 months and using electronic billing systems to report data to Philadelphia's immunization registry in 2001-2003. Chart records were compared to registry records to identify immunizations administered in these practices but not reported to the registry. The study practices administered 256,969 immunizations to 20,611 children. Of these 256,969 administered immunizations, 62,213 (24%) were not in the registry. The electronic billing systems submitted data for all administered immunizations for 69% of immunization visits, some but not all for 11% of visits, and none for 20% of visits. Immunizations administered but not billed cost these providers up to $980,477 in lost revenue from administrative fees alone. Improvement of billing data quality would result in more complete registries, higher reported immunization coverage rates, and recovered revenue for immunization providers.

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