Figure. Assigned Emergency Severity Index (ESI), Version 4, Compared With Algorithm-Derived ESI.

The ESI is a 5-level emergency department triage algorithm that sorts patients into 5 groups from I (most urgent) to V (least urgent). Comparison assumes exact resource and critical care needs were known at triage. Meaningful undertriage was defined as ESI III or IV encounters with level 1 or 2 interventions (ie, should have been ESI I or II, respectively) or ESI V encounters with level 1 to 4 interventions or 2 or more resources used. Meaningful overtriage was defined as ESI II or III encounters that required zero resources prior to ED discharge (ie, should have been ESI V).