Performance of the Emergency Severity Index (ESI) in predicting mortality and resource utilization at a tertiary care hospital in Thailand

Effective triage in the emergency department (ED) is crucial for ensuring timely and appropriate care. The Emergency Severity Index (ESI) is a widely used triage system with strong predictive value for clinical outcomes. However, its performance in middle-income countries remains understudied, particularly in settings with variable resource availability, diverse patient populations, and inconsistent triage practices. This study aimed to assess in-hospital mortality across ESI levels and evaluate the ESI's diagnostic performance in predicting mortality among ED patients in a middle-income country. We conducted a retrospective cohort study at a tertiary care hospital in Thailand, including 331,182 ED patients from January 2016 to December 2021. Data on demographics, ESI level, comorbidities, and outcomes (mortality, ICU admission, and length of stay) were analyzed. The primary outcome was in-hospital mortality across ESI levels 1–5, while secondary outcomes included ICU admission, hospital length of stay, and ED resource utilization. The overall in-hospital mortality rate was 0.6%. However, mortality was significantly higher among patients triaged as ESI level 1 (17%), compared with those at levels 2 (1.7%), 3 (0.3%), 4 (<0.1%), and 5 (0%). ICU admission rates were highest among ESI level 1 patients (35%), followed by those at level 2 (9.8%), with rates declining as triage acuity decreased. ESI levels 1 and 2 demonstrated strong predictive performance for mortality, with a sensitivity of 82.1% and a negative predictive value of 99.9%. Patients classified as ESI level 1 had the highest utilization of diagnostic and therapeutic interventions, including blood tests (93%), supplemental low-flow oxygen (36%), and mechanical ventilation (30%). ESI level 1 was associated with significantly higher in-hospital mortality, ICU admissions, and resource utilization compared with other levels. These findings support the continued use of the ESI for ED triage, especially in middle-income settings.