Emergency Severity Index
The Emergency Severity Index (ESI) is a standardized five-level triage algorithm employed in emergency departments (EDs) to rapidly assess and categorize patients based on the acuity of their presenting condition, physiologic stability, and anticipated resource needs, thereby prioritizing care to minimize morbidity and optimize patient flow.[1] Developed in 1998 by emergency physicians Richard Wuerz and David Eitel, the ESI was initially piloted in 1999 at two U.S. university hospitals and refined through collaborative efforts by the ESI Triage Group, leading to its first formal publication in 2000.[1] By 2023, the fifth edition of the ESI Implementation Handbook had been released by the Emergency Nurses Association (ENA), incorporating updates to address evolving clinical needs and reduce bias in triage decisions.[1] The system is primarily administered by experienced triage nurses during the initial patient encounter, involving a brief history, visual assessment, and vital signs to assign one of five levels: Level 1 for patients requiring immediate life-saving interventions (e.g., cardiac arrest); Level 2 for high-risk situations or confused/lethargic patients; Level 3 for those needing multiple resources (e.g., labs and imaging); Level 4 for minimal resources (e.g., a single exam or treatment); and Level 5 for non-urgent cases requiring no resources beyond the visit itself.[1] As of 2019, the ESI is utilized in approximately 94% of U.S. EDs, making it the most widely adopted triage tool in the country and a benchmark for international adaptations.[2] Its reliability has been validated across multiple studies, demonstrating consistent patient stratification by acuity and resource utilization, though ongoing training and quality monitoring are essential to mitigate risks like undertriage or overtriage.[1] The ESI emphasizes equity in care allocation, particularly for vulnerable populations, and integrates considerations for pain severity, mental health, and social determinants to ensure comprehensive assessments.[1]Background
Development and History
The Emergency Severity Index (ESI) was originally conceived in 1998 by emergency physicians Richard Wuerz and David Eitel as a standardized five-level triage tool to assess patient acuity and anticipated resource needs in emergency departments (EDs). They assembled a collaborative group of ED professionals, including nurses and physicians, to refine the concept, leading to initial pilot testing and implementation in two university-affiliated teaching hospitals in 1999, with further refinement across seven sites by 2000. The Emergency Nurses Association (ENA) played a key role in supporting the tool's standardization starting in 2002 through the first edition of the ESI Implementation Handbook and subsequent policy endorsements, and it acquired ownership of the ESI in 2019 to facilitate ongoing enhancements, resulting in the fourth edition handbook in 2020 and the fifth edition in 2023.[1] The ESI evolved through several versions to improve reliability, clinical applicability, and alignment with resource utilization. Version 1 was introduced in 1999 as the foundational algorithm, followed by Version 2 in 2001, which enhanced inter-rater reliability through clearer decision-making criteria. Version 3, released in 2003, incorporated explicit anticipation of resource needs to better predict patient care requirements. Version 4, refined in 2007 with changes to level 1 criteria and pediatric fever thresholds, was detailed in the 2012 handbook edition, including a dedicated pediatric chapter. Version 5, introduced in the 2023 fifth edition handbook, emphasized vital signs assessment primarily for low-acuity patients (levels 4 and 5) to streamline triage efficiency, alongside revisions to address bias and simplify decision points.[1][3] Key publications documented the ESI's foundational work and validations. The original conceptual framework appeared in a 1999 article by Gilboy, Travers, and Wuerz in the Journal of Emergency Nursing, outlining the triage algorithm's structure. A seminal 2001 paper by Wuerz et al. in Academic Emergency Medicine detailed the implementation and refinement process across multiple sites, establishing early reliability metrics. Subsequent validations, such as those in Annals of Emergency Medicine, confirmed the tool's predictive accuracy for resource consumption and patient outcomes. Adoption of the ESI accelerated in the early 2000s, with widespread implementation in U.S. EDs by 2005, driven by endorsements from professional organizations. A 2005 report from the American College of Emergency Physicians (ACEP) and ENA five-level triage task force specifically recommended the ESI as a validated system, aligning it with emerging national standards. Around the same time, The Joint Commission integrated requirements for reliable triage processes into its accreditation standards, facilitating the ESI's incorporation as a compliant tool in hospital protocols. As of 2025, a simulation study has affirmed Version 5's ongoing relevance, demonstrating its predictive validity for triage level distribution and associations with hospitalization and ED outcomes comparable to prior versions, with minimal shifts in overall patient categorization.[4]Purpose and Principles
The Emergency Severity Index (ESI) serves as a standardized triage tool designed to rapidly assess and sort patients presenting to emergency departments (EDs) based on acuity, thereby prioritizing those requiring immediate interventions to mitigate risks in overcrowded settings. Its primary objectives include predicting the need for resources such as laboratory tests, imaging, or consultations, which helps optimize patient flow, reduce wait times for high-acuity cases, and ensure timely care to prevent adverse outcomes like morbidity or mortality.[1] Developed initially in 1999 by emergency physicians Richard Wuerz and David Eitel, the ESI addresses the challenges of increasing ED volumes by providing a reliable method to balance workload and staffing needs.[5] At its core, the ESI operates on a five-level scale that integrates clinical severity—focusing on immediate threats to life or limb—with anticipated resource consumption, distinguishing it from purely acuity-based systems by estimating whether a patient will require minimal (e.g., simple interventions) or multiple (e.g., complex diagnostics and treatments) resources. This design philosophy employs a decision-tree algorithm to promote consistency and reproducibility in triage decisions, enabling quick categorization without relying solely on subjective judgments. The system's principles emphasize physiologic stability as the initial assessment criterion, followed by resource evaluation for stable patients, ensuring that triage reflects both urgency and operational demands rather than predefined wait times.[1] The ESI is targeted primarily for use in U.S. EDs with both adult and pediatric patients, though it has been adapted for prehospital settings by emergency medical services personnel to facilitate early prioritization during transport. It relies heavily on nurse-led triage, leveraging the clinical judgment of registered nurses experienced in emergency care, while incorporating objective vital signs as anchors to enhance reliability and validity across diverse populations.[1] As of 2019, the ESI is used by over 94% of U.S. EDs, differentiating itself from acuity-only triage models, such as the Australasian Triage Scale, by explicitly accounting for resource utilization, which better supports workload distribution and resource allocation in high-volume environments.[1][6][2]Triage Methodology
Algorithm Overview
The Emergency Severity Index (ESI) is a standardized five-level triage algorithm designed for emergency departments to rapidly categorize patients based on acuity and anticipated resource needs. The algorithm follows a sequential, decision-tree structure that begins with identifying patients requiring immediate lifesaving interventions and progresses through assessments of risk, resource utilization, and vital signs to assign levels 1 through 5. This process integrates principles of acuity-based sorting, where higher levels indicate greater urgency and resource demands, enabling efficient patient prioritization in high-volume settings.[1] The flowchart logic commences with Step A: "Does this patient require immediate lifesaving intervention?"—addressing those with immediate life threats who need instantaneous intervention, such as advanced resuscitation (Level 1 if yes). If not, it advances to Step B: "Is this a high-risk situation, or is the patient in severe pain or distress, or confused/lethargic/disoriented?" directing to Level 2 if affirmative. For remaining patients, Step C involves anticipating resource needs—differentiating between no resources (Level 5), one simple resource (Level 4), or two or more different types of resources (Level 3). Step D then checks for high-risk vital signs, potentially reassessing and up-triaging acuity if abnormalities are present. These branches rely on yes/no decision points to streamline triage, typically completed by nurses in 2-5 minutes during the initial encounter.[1] The algorithm balances subjective nurse judgment with objective data, allowing triage personnel to incorporate clinical intuition—such as whether a patient "looks sick"—alongside measurable elements like vital signs and projected interventions. This hybrid approach ensures holistic assessments without over-relying on any single factor, promoting reliability across diverse patient presentations. The standard ESI algorithm diagram, as depicted in the Version 5 handbook, illustrates this as a linear flowchart with labeled steps (A through D), branching arrows for yes/no outcomes, and clear pathways to each acuity level, serving as a visual guide for consistent application.[1]Assessment Criteria
The assessment criteria in the Emergency Severity Index (ESI) triage process involve a systematic evaluation of patient acuity through vital signs, situational risks, anticipated resource needs, and subjective indicators to determine urgency. These criteria are applied by triage nurses following a decision tree that begins with identifying immediate life threats and progresses to resource estimation.[1] Vital signs serve as a core objective measure in ESI, with abnormal parameters signaling potential instability and prompting reassessment at Step D. In Version 5, high-risk vital signs are limited to heart rate, respiratory rate, and oxygen saturation. For adults (>18 years), high-risk vital signs include heart rate greater than 100 beats per minute, respiratory rate greater than 20 breaths per minute, and oxygen saturation less than 92% on room air. Blood pressure and temperature are not included as high-risk vital signs for adults.[1] Pediatric thresholds are adjusted for age to account for physiological differences, with the same parameters (heart rate, respiratory rate, oxygen saturation <92%). Specific age bands are: heart rate >190 bpm (<1 month), >180 bpm (1-12 months), >140 bpm (1-3 years), >120 bpm (3-5 years and 5-12 years), >100 bpm (12-18 years); respiratory rate >60 breaths/min (<1 month), >55 breaths/min (1-12 months), >40 breaths/min (1-3 years), >35 breaths/min (3-5 years), >30 breaths/min (5-12 years), >20 breaths/min (12-18 years).[1] High-risk situational criteria focus on clinical presentations that could rapidly deteriorate, regardless of vital signs. Examples include unstable chest pain suggestive of acute coronary syndrome, new-onset altered mental status, and end-of-life issues such as imminent death or uncontrolled symptoms in terminal illness.[1] These criteria prioritize patients with conditions like suicidal ideation or acute behavioral changes that demand immediate intervention.[1] Subjective assessments identify high-risk patients through history and presentation, including age extremes such as children under 5 years or adults over 85 years presenting with concerning complaints like falls or weakness, as well as confusion, disorientation, or intoxication that impairs reliable communication.[1] These factors help triage nurses recognize vulnerabilities not captured by vital signs alone, such as in elderly patients with atypical symptoms of serious illness.[1] Resource anticipation evaluates the expected interventions needed, where "resources" are defined as simple procedures like laboratory tests, intravenous access, medications, imaging studies, or specialty consultations.[1] For instance, patients anticipated to require two or more different types of resources—such as blood work and a computed tomography scan—are assigned Level 3.[1] Pain assessment is integrated using standardized numeric scales, typically the 0-10 rating where 0 indicates no pain and 10 the worst imaginable, to quantify severity and inform risk stratification.[1] A pain score of 7/10 or greater, corroborated by clinical observation of distress, should be considered for Level 2 assignment, particularly when indicative of systemic issues.[1] For pediatric fever, which influences acuity but is separate from high-risk vital signs: assign at least Level 2 if temperature >100.4°F (38°C) for 1-28 days; consider Level 2 if >100.4°F (38°C) for 1-3 months; consider Level 2 or 3 if >102.2°F (39°C) or <96.8°F (36°C) for ≥3 months, especially with incomplete immunizations or no identified source.[1]| Vital Sign | Adult Thresholds (>18 years) | Pediatric Thresholds (Age-Adjusted, <18 years) |
|---|---|---|
| Heart Rate | >100 bpm | <1 mo: >190 bpm 1–12 mo: >180 bpm 1–3 y: >140 bpm 3–5 y: >120 bpm 5–12 y: >120 bpm 12–18 y: >100 bpm |
| Respiratory Rate | >20 breaths/min | <1 mo: >60 breaths/min 1–12 mo: >55 breaths/min 1–3 y: >40 breaths/min 3–5 y: >35 breaths/min 5–12 y: >30 breaths/min 12–18 y: >20 breaths/min |
| Oxygen Saturation | <92% on room air | <92% on room air (all ages) |