Background Hospital-admitted patients are at risk of certain outcomes during hospital stay. They include admission to intensive care units (ICUs), placement on the ventilator and complications. Comorbidities could increase these risks. The Charlson Comorbidity Index (CCI) and the Elixhauser Comorbidity Measure (ECM), originally derived for mortality outcomes among general medical populations, are widely used for these in-hospital complications among injury populations. This study derived indices to specifically capture the effect of comorbidity on in-hospital complications for injury patients.

Methods Retrospective data on injury hospital-admissions from July 2012 to June 2014 (161,334 patients) for the state of Victoria, Australia was analysed. Results from multivariable regression analysis were used to derive the Australian Injury Comorbidity Indices (AICIs) for ICU hours, ventilator hours and hospital-acquired complications. The AICIs, CCI and ECM were validated on data from Victoria and two other Australian states.

Results Five comorbidities were significantly associated with ICU hours, two with ventilator hours and fifteen with complications. Not all diseases listed in the CCI or ECM were found to be associated with these outcomes. The AICIs performed equally well in terms of predictive ability to the long-listed ECM and in most instances outperformed the CCI.

Conclusions Associations between outcomes and comorbidities vary based on the type of outcome measure.

Learning Outcomes New comorbidity indices developed in this study provide a relevant, parsimonious and up-to-date capture of associations between comorbidities and in-hospital complications among injury patients, and is better suited for use in this context compared to the CCI and ECM.