Background Knowledge of fatal injuries is required to inform prevention activities. Where hospital patients with an injury principal diagnosis (PDx) died and were certified to a medical underlying cause of death (UCoD), there is the potential to underestimate injury mortality. We sought to characterise injury/non-injury (NI) mismatches between PDx and UCoD by identifying which subgroups had small/large mismatches, and to understand why mismatches had occurred using informative examples.

Method Hospital records (n=10 234) with a PDx of injury were linked to the mortality collection using a unique personal identifier. Percentages UCoD coded to a NI were tabulated, for three follow-up periods and by selected variables. Additionally, we reviewed a sample of 70 records for which there was a mismatch.

Results %NIs were 39%, 66% and 77% for time from injury to death of <1 week, <90 days and <1 year, respectively. Variations in %NI were found for all variables. Illustrative examples of 70 medical UCoD deaths showed that for 12 cases the injury event was unequivocally judged to have resulted in premature death. A further 16 were judged as injury deaths using balance of probability arguments.

Conclusion There is variation in rates of mismatch between PDx of injury and UCoD of NI. While legitimate reasons exist for mismatches in certain groups, a material number of injury deaths are not captured using UCoD alone; a new operational definition of injury death is needed. Early solutions are proposed. Further work is needed to investigate operational definitions with acceptable false positive and negative detection rates.