Written by Aaron Lacy
Spoon Feed
This review advocates for a risk-stratified observation pathway in those who receive epinephrine for anaphylaxis, from as little as 1–3 hours if low-risk to as high as 6–12 hours if high-risk.
Hurry up and wait… for that biphasic reaction?
When you see anaphylaxis, you give 0.3–0.5mg of intramuscular epinephrine. You just saved a life, but that was the easy part. Now the hard part—how long do you watch the patient in your busy ED with a waiting room bursting at the seams?
This narrative review critically appraised the evidence on observation after epi for anaphylaxis from the last 10 years. Biphasic reactions are a real threat and usually occur after a 4- to 6-hour window. Pooled meta-analysis data shows a negative predictive value for a biphasic reaction of 95% at 1 hour, 97% at >6 hours, and >98% for periods exceeding 8–12 hours. The authors highlight that after a 6-hour period of observation, there is an estimated miss rate of 27 cases per 1,000 discharged patients. However, this is a pooled approach, and the answer of how long to observe can and should be more nuanced.
The authors advocate for a risk-stratified approach (as do the newest guidelines from the AAAAI and ACAAI) based on initial severity of presentation.
Low-risk patients are those with rapid resolution of symptoms with one dose of epi, exposure to a known food trigger, no significant comorbidities or severe features (e.g., hypotension or hypoxia), and a safe discharge plan. These patients can be observed for 1–3 hours.
High-risk patients are those who require two or more doses of epi (the most consistent predictor of biphasic reactions) and have severe features, unknown or non-food trigger, comorbid conditions (including mast cell disorders), or unsafe discharge plan. These patients should be observed for a minimum of 6 hours and watched up to 12 hours or be admitted.
How will this change my practice?
In training, I was taught to observe these patients for 4–6 hours. From my own personal dives into the anaphylaxis literature (and especially after this article), I think this is too long for low-risk patients and not long enough for high-risk patients. This article provides parameters for who I can sort into my low-risk category (and discharge faster) versus my high-risk category (observe for longer or admit).
Source
Post-anaphylaxis observation in the ED: a decade of data challenging the traditional 24-hour rule. Int J Emerg Med. 2026 Mar 14;19(1):63. doi: 10.1186/s12245-026-01175-4. PMID: 41832433; PMCID: PMC12990544.
