Written and maintained by CASRAI Editorial Board
Last updated
The Emergency Severity Index (ESI) is a five-level emergency department (ED) triage algorithm that stratifies incoming patients simultaneously by clinical acuity (how soon the patient must be seen by a clinician) and predicted resource consumption (how many distinct diagnostic or therapeutic interventions will be needed to reach an emergency department disposition). Originally developed by emergency physicians Richard Wuerz and David Eitel, researched and published by the Agency for Healthcare Research and Quality (AHRQ), and maintained by the Emergency Nurses Association (ENA) through Version 4 and Version 5, the ESI is the most widely implemented emergency triage standard in United States hospitals and clinical informatics systems.
What Is the Emergency Severity Index (ESI) and Its 5 Triage Levels?
Unlike earlier three-level triage systems (emergent, urgent, non-urgent) that suffered from low inter-rater reliability across triage nurses, the ESI assigns every emergency department arrival to one of five unambiguous numerical levels from ESI Level 1 (highest urgency) to ESI Level 5 (lowest urgency):
| ESI Triage Level | Clinical Acuity & Resource Criterion | Governing Decision Point | Representative Clinical Presentations |
|---|---|---|---|
| ESI Level 1 | Requires immediate life-saving intervention | Decision Point A (Yes) | Cardiac or respiratory arrest, apnea, severe respiratory distress requiring immediate intubation, unresponsive patient with Glasgow Coma Scale ≤ 8, profound shock |
| ESI Level 2 | High-risk presentation, new-onset confusion/lethargy/disorientation, severe pain/distress, or Danger Zone vital signs | Decision Point B (Yes) or Decision Point D upgrade | Acute chest pain suggestive of ACS, acute focal neurologic deficit (Cincinnati Prehospital Stroke Scale / NIH Stroke Scale alert), active suicidal ideation, immunocompromised patient with fever |
| ESI Level 3 | Stable vital signs; predicted to require two or more distinct ED resources | Decision Point C (Many) + Decision Point D (Normal) | Right lower quadrant abdominal pain with stable vitals requiring blood/urine labs, IV fluid hydration, and an abdominal CT scan |
| ESI Level 4 | Predicted to require exactly one ED resource | Decision Point C (One) | Uncomplicated ankle sprain requiring a single extremity X-ray, or a simple superficial laceration requiring suture repair only |
| ESI Level 5 | Predicted to require zero ED resources beyond history and physical exam | Decision Point C (None) | Medication prescription refill request, minor abrasion requiring only a bandage, uncomplicated viral upper respiratory symptoms |
The 4 Sequential Decision Points of the ESI Algorithm (A, B, C, and D)
The ESI triage algorithm operates as a four-step conceptual flowchart evaluated sequentially by an ESI-trained triage nurse upon patient arrival:
Decision Point A: Does the Patient Require Immediate Life-Saving Intervention?
The triage nurse first evaluates airway, breathing, circulation, and responsiveness. If the patient requires an immediate intervention aimed at securing the airway, maintaining breathing, supporting circulation, or correcting a major neurological compromise—such as bag-valve-mask ventilation, endotracheal intubation, defibrillation, emergent cardioversion, chest needle decompression, intraosseous access, or immediate IV push of naloxone, dextrose, or vasopressors—the patient is assigned ESI Level 1 and moved directly to a resuscitation bay without further triage questioning.
Decision Point B: Should the Patient Wait?
If the patient does not require an immediate life-saving intervention at Decision Point A, the nurse asks whether the patient can safely wait for the last open treatment bed using three criteria:
- Is this a high-risk situation? A presentation with a high likelihood of rapid clinical deterioration or a time-critical intervention window (e.g., suspected acute coronary syndrome, acute stroke, ectopic pregnancy, testicular torsion, sepsis, or severe postpartum hemorrhage).
- Is the patient confused, lethargic, or disoriented? Any acute alteration in mental status from the patient’s baseline.
- Is the patient in severe pain or distress? Assessed via clinical observation, physiological indicators, and patient self-report alongside urgency of needed intervention.
If the answer to any of these three criteria is yes, the patient is assigned ESI Level 2.
Decision Point C: How Many Different Resources Are Needed?
For patients who do not meet Level 1 or Level 2 acuity criteria, the triage nurse estimates how many different types of ED resources will be required for the emergency physician or advanced practice provider to reach a disposition decision (discharge, observation, inpatient admission, or transfer). Under the AHRQ/ENA ESI rules, resource counting measures distinct resource categories, not individual test orders:
| Counts as an ESI Resource | Does NOT Count as an ESI Resource |
|---|---|
| Clinical laboratory testing (blood and urine panels together count as 1 resource; e.g., CBC + BMP + urinalysis = 1 resource) | History and physical examination (including pelvic, rectal, or slit-lamp exam) |
| Diagnostic imaging by modality (each distinct modality counts as 1 resource: e.g., chest X-ray + ankle X-ray = 1 resource; chest X-ray + head CT = 2 resources) | Point-of-care testing at bedside (fingerstick blood glucose, urine dip, gastroccult) |
| Intravenous (IV) fluids (hydration) | Saline or heparin lock insertion alone |
| IV, intramuscular (IM), or nebulized medications | Oral (PO) medications, tetanus immunization, or prescription refills |
| Specialty consultation (e.g., surgical or cardiology consult) | Phone call to the patient’s primary care physician |
| Simple procedure = 1 resource (e.g., laceration repair, Foley catheter); Complex procedure = 2 resources (e.g., procedural sedation) | Simple wound care (bandages, slings, crutches, splints) |
If predicted resources equal 0, the patient is assigned ESI Level 5. If predicted resources equal 1, the patient is assigned ESI Level 4. If predicted resources equal 2 or more, the nurse proceeds to Decision Point D before finalizing Level 3.
Decision Point D: What Are the Patient’s Vital Signs? (Danger Zone Thresholds)
Before assigning ESI Level 3 to a patient requiring two or more resources, the triage nurse must obtain and evaluate vital signs—specifically heart rate (HR), respiratory rate (RR), oxygen saturation ($ ext{SpO}_2$), and temperature for young children. Under the standard ESI Danger Zone criteria (for adults: $text{HR} > 100$ bpm, $text{RR} > 20$ breaths/min, or $text{SpO}_2 < 92%$, alongside age-stratified pediatric vital sign cutoffs), if any vital sign exceeds the Danger Zone threshold, the nurse must consider upgrading the patient from Level 3 to ESI Level 2.
How the Emergency Severity Index Is Used in Clinical Research and Health Informatics
In emergency medicine health services research, hospital quality benchmarking (including AHRQ Patient Safety Indicators and CMS emergency department throughput measures), and EHR clinical decision support, the ESI serves three vital roles:
- Standardized Acuity Adjustment in Observational Research: National datasets such as the CDC National Hospital Ambulatory Medical Care Survey (NHAMCS) record ESI levels to control for baseline arrival acuity when studying ED crowding, boarding times, left-without-being-seen (LWBS) rates, and inpatient mortality.
- Under-Triage, Over-Triage, and Equity Auditing (ESI Version 5): Because more than half of all US emergency department visits are initially categorized as ESI Level 3, ENA’s Version 5 updates emphasize objective risk stratification and mitigation of implicit cognitive and demographic biases that historically caused higher under-triage rates among pediatric, geriatric, and minority patient populations.
- Electronic Triage (e-Triage) and Machine Learning CDS: Modern hospital EHRs embed rule-based ESI calculators and machine learning clinical decision support (CDS) tools that ingest real-time vital signs, chief complaints, and historical EHR comorbidities to alert triage nurses when a tentative ESI Level 3 patient carries a high predicted risk of ICU admission or sepsis, reducing mistriage.
