METHODOLOGIC ISSUES Reporting of the incidence of hospitalised injuries: numerator issues

Objectives: To examine and discuss the implications on the incidence of hospitalised injuries of selecting cases from principal diagnosis field only compared with considering all diagnosis fields, the inclusion compared with the exclusion of medical injuries, and the impact of identifying multiple admissions. Methods: Analysis of data from the 1999–2000 New South Wales Inpatient Statistics Collection, Australia, including an internal linkage of the same dataset. Results: Approximately 27.5% of records with a non-injury primary diagnosis include a nature of injury diagnosis in a subsequent diagnostic field. This figure increased to more than half (53%) of discharges for medical injuries. The internal linkage showed that 6.5% of discharges were repeat admissions for the same International Classification of Diseases, 10th revision (ICD-10) injury code and that 13.8% were repeat admissions for any ICD-10 injury code. The proportions of repeat admissions varied according to the type and the mechanism of injury. Conclusions: Selecting hospitalised injury cases from the principal diagnosis alone would underestimate medical injury cases as well as other injuries occurring in hospital. Repeat admissions should always be considered particularly in the case of thermal injuries, self harm, and medical injuries. Due to the limitations of data linkage, alternative methods need to be developed to identify repeat admissions. Other areas in which further research would be beneficial to a more uniform reporting of injury hospitalisations include better identification of injuries occurring in hospital, a review of ICD-10 injury codes, and the development an ICD-10 based severity measure which can be readily used with hospital discharge data.