Validation of the Norwegian survival prediction model in trauma (NORMIT) in Swedish trauma populations.

Background: Trauma survival prediction models can be used for quality assessment in trauma populations. The Norwegian survival prediction model in trauma (NORMIT) has been updated recently and validated internally (NORMIT 2). The aim of this observational study was to compare the accuracy of NORMIT 1 and 2 in two Swedish trauma populations.

Methods: Adult patients registered in the national trauma registry during 2014-2016 were eligible for inclusion. The study populations comprised the total national trauma (NT) population, and a subpopulation of patients admitted to a single level I trauma centre (TC). The primary outcome was 30-day mortality. Model validation included receiver operating characteristic (ROC) curve analysis and GiViTI calibration belts. The calibration was also assessed in subgroups of severely injured patients (New Injury Severity Score (NISS) over 15).

Results: A total of 26 504 patients were included. Some 18·7 per cent of patients in the NT population and 2·6 per cent in the TC subpopulation were excluded owing to missing data, leaving 21 554 and 3972 respectively for analysis. NORMIT 1 and 2 showed excellent ability to distinguish between survivors and non-survivors in both populations, but poor agreement between predicted and observed outcome in the NT population with overestimation of survival, including in the subgroup with NISS over 15. In the TC subpopulation, NORMIT 1 underestimated survival irrespective of injury severity, but NORMIT 2 showed good calibration both in the total subpopulation and the subgroup with NISS over 15.

Conclusion: NORMIT 2 is well suited to predict survival in a Swedish trauma centre population, irrespective of injury severity. Both NORMIT 1 and 2 performed poorly in a more heterogeneous national population of injured patients.