The Emergency Severity Index (ESI) is a five-level tool for use in emergency department (ED) triage. Experienced ED nurses use the ESI to rate patient acuity, from level 1 (most urgent) to level 5 (least resource intensive). The ESI is unique among triage tools, by including both acuity and resource needs in the system of categorizing ED patients.

The ESI is a powerful tool for enhancing patient safety at triage as well as providing casemix data to support emergency department operational decisions, quality initiatives and clinical research.