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Use of mixed methods to investigate case definitions to improve the identification of serious injury cases from hospital episode data
  1. Colin Cryer1,
  2. Gabrielle Davie1,
  3. Pauline Gulliver2,
  4. Ari Samaranayaka3
  1. 1 Injury Prevention Research Unit, Department of Preventive and Social Medicine, University of Otago–Dunedin Campus, Dunedin, New Zealand
  2. 2 School of Population Health, The University of Auckland, Auckland, New Zealand
  3. 3 Centre for Biostatistics, University of Otago, Dunedin, New Zealand
  1. Correspondence to Dr Colin Cryer, Injury Prevention Research Unit, Department of Preventive and Social Medicine, University of Otago–Dunedin Campus, Dunedin 9054, New Zealand; colincryer{at}


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.

  • surveillance
  • indicators
  • mixed methods
  • health services

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  • Contributors All authors were members of the team that managed the project on which this paper is based. They contributed to its design, steered its execution and advised on the analysis and reporting. CC made the first draft of the paper which was revised incrementally by each author to produce the final manuscript.

  • Funding The report on which this paper is based was commissioned and funded by Official Statistics Research, through Statistics New Zealand.

  • Disclaimer The funder had no influence on the study design, the collection, analysis and interpretation of data, the writing of the report, or the decision to submit the paper for publication. The opinions, findings, recommendations and conclusions expressed in this paper are those of the authors, and do not necessarily represent those of Statistics New Zealand.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval The University of Otago has ongoing ethical approval to conduct research using NMDS data for the purpose of monitoring injury at a national level (NZ Health and Disability Ethics Committee OTA/99/02/008/AM06).

  • Provenance and peer review Not commissioned; externally peer reviewed.