Background: Trauma team activation at a Level 1 trauma centre In Quebec, Canada, is primarily at the emergency department’s staff discretion. Trauma teams may be activated prehospital, based on information provided by field paramedics or in the emergency department based on the patient’s condition on arrival. In this study, we examined over and undertriage rates based on present trauma team activation criteria. We also examined if trauma team activation, for those patients solely meeting pre-hospital major trauma criteria, would result in significant overactivation of the trauma team. Methods: This is a single-centre retrospective medical record review. Primary ambulance transport reports from May 15, 2018 to December 31, 2020, were screened to identify patients aged ≥16 years who met pre-hospital trauma triage criteria to bypass community hospitals to arrive directly at the Level 1 trauma centre. We examined pre-hospital triage criteria, trauma team involvement, Injury Severity Scores (ISS) and final disposition. Patients were evaluated for over and undertriage and rates were compared to the rates assuming all patients were to have a trauma team activation. We considered patients overtriaged if they had a full trauma team activation but had an ISS <12 and were discharged from the emergency department. Undertriage was defined as any patient with an ISS ³12 and did not have a trauma team activation. Results: Of the 371 patients who met study inclusion criteria, 123 (33.3%) did not meet trauma team activation criteria, while 214 (57.7%) had a trauma team activation. Of these, 49 patients (13.2%) were undertriaged and 31 patients (8.4%) were overtriaged and 25.8% of the major trauma patients (ISS ³ 12) were undertriaged. A trauma team activation for all meeting field triage criteria increased overtriage to 25.3% and brought undertriage rates to 0%, with statistically significant differences based on the Wilcoxon signed ranks test (p<0.05). Conclusions: In this study, undertriage rates were well above 5%. Trauma team activation, based on local field trauma triage criteria adapted from the CDC-ACSCOT field triage criteria, eliminates undertriage and keeps overtriage rates below 35%. This research suggests that field triage criteria accurately predict major trauma and the need for the involvement of the trauma team and that the condition of the trauma patient in the pre-hospital setting is accurately predicting the need for advanced trauma care