Use of mixed methods to investigate case definitions to improve the identification of serious injury cases from hospital episode data.

Introduction: It has been commonplace internationally, when using hospital data, to use the principal diagnosis to identify injury cases and the first external cause of injury code (E-code) to identify the main cause. Our purpose was to investigate alternative operational definitions of serious non-fatal injury to identify cases of interest for injury surveillance, both overall and for four common causes of injury.

Methods: Serious non-fatal injury cases were identified from New Zealand (NZ) hospital discharge data using an alternative definition: that is, case selection using principal and additional diagnoses. Separately, identification of cause used all E-codes on the discharge record. Numbers of cases identified were contrasted with those captured using the usual definition. Views of NZ government stakeholders were sought regarding the acceptability of the additional cases found using these alternative definitions. Views of international experts were also canvassed.

Results: When using all diagnoses there was a 7% increase in 'all injury' cases identified, a 17% increase in self-harm cases and 8% increase in falls cases. Use of all E-codes resulted in a 4% increase in self-harm cases, 2% increase in assault cases and 1% increase in both falls and motor vehicle traffic crash cases.

Discussion: A case definition based solely on principal diagnosis fails to count a material number of serious non-fatal injury cases that are of interest to the injury prevention community. There is a need, therefore, to use an alternative case definition that includes additional diagnoses. Use of multiple E-codes to classify cause of injury should be considered.