A trauma mortality prediction model based on the anatomic injury scale

Abstract

Objective: To develop a statistically rigorous trauma mortality prediction model based on empiric estimates of severity for each injury in the abbreviated injury scale (AIS) and compare the performance of this new model with the injury severity score (ISS).

Summary background data: Mortality rates at trauma centers should only be compared after adjusting for differences in injury severity, but no reliable measure of injury severity currently exists. The ISS has served as the standard measure of anatomic injury for 30 years. However, it relies on the individual injury severities assigned by experts in the AIS, is nonmonotonic with respect to mortality, and fails to perform even as well as a far simpler model based on the single worst injury a patient has sustained.

Methods: This study is based on data from 702,229 injured patients in the National Trauma Data Bank (NTDB 6.1) hospitalized between 2001 and 2005. Sixty percent of the data was used to derive an empiric measure of severity of each of the 1322 injuries in the AIS lexicon by taking the weighted average of coefficients estimated using 2 separate regression models. The remaining 40% of the data was use to create 3 exploratory mortality prediction models and compare their performance with the ISS using measures of discrimination (C statistic), calibration (Hosmer Lemeshow statistic and calibration curves), and the Akaike information criterion.

Results: Three new models based on empiric AIS injury severities were developed. All of these new models discriminated survivors from nonsurvivors better than the ISS, but one, the trauma mortality prediction model (TMPM), had both better discrimination [ROCTMPM = 0.901 (0.898-0.905), ROCISS = 0.871 (0.866-0.877)] and better calibration [HLTMPM = 58 (35-91), HLISS = 296 (228-357)] than the ISS. The addition of age, gender, and mechanism of injury improved all models, but the augmented TMPM dominated ISS by every measure [ROCTMPM = 0.925(0.921-0.928), ROCISS = 0.904(0.901-0.909), HLTMPM = 18 (12-31), HLISS = 54 (30-64)].

Conclusions: Trauma mortality models based on empirical estimates of individual injury severity better discriminate between survivors and nonsurvivors than does the current standard, ISS. One such model, the TMPM, has both superior discrimination and calibration when compared with the ISS. The TMPM should replace the ISS as the standard measure of overall injury severity.