Assessing the Impact of Waiting Time on Triage Color Code Assignment and One-Year Mortality in the Emergency Department: A Causal Mediation Analysis.

Background and Aims: Emergency Department (ED) overcrowding and delays in care affect patient outcomes. While triage systems prioritize care based on urgency, the role of waiting time in mediating the relationship between triage color codes and 1-year mortality remains unclear. This study investigates this mediation effect to improve triage protocols and patient outcomes. Methods: A retrospective cohort study was conducted using data from the Fondazione Policlinico Universitario Agostino Gemelli IRCCS ED (2014-2018). The sample included patients assigned green and yellow triage codes, excluding red and white ones. The outcome was 1-year mortality; the mediator was waiting time, defined as the delay between triage assignment and medical evaluation. Causal mediation analysis estimated direct, indirect, and total effects, with sensitivity analyses assessing robustness to unmeasured confounding. Results: Among 56,284 observations, older patients and yellow-coded individuals showed higher 1-year mortality. Waiting time did not significantly mediate the relationship between triage code and mortality (ACME OR: 1.001, 95% CI: 0.999-1.002). Triage code, however, had a direct significant effect on mortality (ADE OR: 1.01, 95% CI: 1.004-1.007). Waiting time mediated a small proportion of the effect (3.4%-13.9%). Sensitivity analyses indicated the mediation effect was sensitive to unmeasured confounding. Conclusions: Triage color code strongly predicts 1-year mortality, independent of waiting time within standard thresholds. For lower-acuity cases, reducing waiting time further may not improve long-term outcomes. Future research should validate these findings across multicenter settings and explore Italy's updated five-color triage system to optimize care delivery.