The Emergency Severity Index, ESI, is a triage system. That means it is a set of rules that healthcare providers use to prioritize care. Though many triage systems exist, ESI is designed specifically for use in hospital emergency departments, and is the one most often used by hospitals in the United States.

Under ESI rules, every patient who shows up is assigned an acuity level from 1 to 5, with level 1 being the most urgently in need of care. Patients are seen in order of acuity, so a person having a heart attack doesn't have to wait in line behind someone with a paper cut.

Since triage is a fast process that does not take the time for a head-to-toe assessment, levels are broad categories based on initial presentation. They are assigned based on the apparent immediacy of the case and, for lower levels, the expected amount of resources required in evaluation or treatment. (The rationale is that patients who need more care also need it sooner.) Here, "resource" means a process that takes a significant amount of equipment or skilled intervention, such as lab tests, radiological imaging, laceration repair, or the like. Basic physical examination or routine nursing care are explicitly not counted as resources, because all patients receive those in all cases.

The full ESI definition runs to dozens of pages, but the categories can be generally summed up as follows.

Level 1: The patient is dying. Immediate life-sustaining intervention is necessary. Drop everything else and come a'running, because without help the patient's condition will be incompatible with life. Such a patient will usually come in via ambulance with care already underway. Examples: Cardiac arrest. Traumatic amputation. Penetrating brain injury. Severe acute asthma attack. An infant found limp and unresponsive.

Level 2: The patient should be seen immediately if at all possible. They are not trying to die at this moment, but are in an unstable situation and are at high risk of having their condition degrade. Delay of care could place them in danger of permanent harm to life, limb, or organ. Examples: Sudden onset confusion and one-sided paralysis. Moderate asthma attack. Stated suicidal impulses. Full thickness burn to the lower extremity.

(Note that many walk-in patients will be judged to meet level 2. Patients who appear stable to a layperson may need to be seen immediately to rule out serious hidden problems. Certain kinds of sudden back pain are probably from a pulled muscle, but might be from a dissecting aneurysm. Certain kinds of abdominal pain could be either menstrual cramps or ectopic pregnancy. These are the trickiest patients to categorize, requiring lots of experience and the exercise of a nurse's sixth sense.)

Level 3: The patient can wait to be seen, but will require in-depth evaluation, defined as using two or more resources in testing or treatment. Patients expected to require more than one type of resource, but who have vital signs in the normal range and who are not at high risk of immediate deterioration, are assigned level 3. Examples: Right lower quadrant abdominal pain, requiring blood tests and imaging to rule out appendicitis. Elderly patient with fever and coughing up thick green phlegm, requiring at least a nebulizer treatment and a chest x-ray. Flank pain consistent with kidney stone, requiring urinalysis, CT scan, and IV pain medication.

Level 4: Similar to level 3, but clearly stable and expected to need only one type of resource. Examples: Mild dehydration needing intravenous fluids. A clean, minor laceration that needs sutures. Apparent urinary tract infection needing a urinalysis. Apparent ankle sprain needing an x-ray to rule out fracture.

Level 5: Stable, with no resources required except physical examination, routine care, oral or topical medication, and/or written prescriptions. Examples: Small abrasion that needs a gauze dressing. Child with earache but no other symptoms. Dental pain with no injury. Prescription refill.

Note that although these levels do indicate which patients tend to be more work for the staff, that is only a side effect, and department workload is not to be considered during the triage assessment. Also, remember that these levels are only a "first look" approximation. If a patient is initially triaged at Level 5 but is found to be in worse shape than expected, of course they are still provided all the care and resources appropriate to their condition.

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