Clinical Conundrums: How Long Should We Monitor After Giving IM Epinephrine for Anaphylaxis?

How Long Should We Monitor After Giving IM Epinephrine for Anaphylaxis?

Bottom Line Up Top: After prompt recognition and appropriate treatment with IM epinephrine, the risk of biphasic reactions are exceedingly low. There is no set observation time to monitor patients as long as their symptoms have resolved. At the time of discharge, appropriate patient education and prescriptions for IM epinephrine are essential.

Clinical Scenario: A 15-year-old male with a history of multiple food allergies presents after ingestion of a sandwich during a school gathering. The patient complains of throat discomfort, rash, and vomiting immediately after ingestion of the sandwich. You note on exam an urticarial rash but no signs of mucosal swelling or respiratory distress. His vital signs are normal. The patient is given 0.5 mg of intramuscular (IM) epinephrine for anaphylaxis. His symptoms are completely resolved after treatment.The patient asks if he can go back to his school gathering now that he feels better.

What Your Gut Says: He can’t leave now, it’s only been an hour. There is a chance of biphasic reactions and I haven’t observed him long enough to ensure a second reaction does not occur.

What The Evidence Says: Prompt recognition of anaphylaxis is imperative. Even after appropriate treatment with IM epinephrine, there is a potential risk of a recurrence of anaphylaxis without exposure to a triggering antigen. This is called a biphasic reaction. The risk of a biphasic reaction is what keeps patients in the ED while being observed for a set period of time. The overall incidence of biphasic reactions is unknown with rates quoted from < 0.5% up to 23% (Lieberman 2005, Rohacek 2014, Tole 2007, Grunau 2014). This varied incidence can be due to inconsistent definitions or inclusion of mild reactions. Since half of biphasic reactions occur within the first 6-12 hrs (Lee 2014) physicians often choose an observation time of 4-6 hrs. Data from a more relevant study that focuses on clinically significant biphasic reactions shows that the incidence is exceedingly low and the reaction may even occur several days post discharge (Grunau 2014).

Which patients are at risk for experiencing biphasic reactions? While the evidence is not strong, those who have been undertreated, experienced delays in treatment, or required multiple injections of epinephrine are more prone to experience a recurrence of symptoms (Ellis 2007, Liu 2020). Thus, if the patient has complete resolution of symptoms after appropriate treatment of anaphylaxis, there is no set time period for monitoring in the ED. At the time of discharge, thorough education and the availability of IM epinephrine for the patient is paramount to allow for prompt treatment of an unlikely biphasic reaction (Pourmand 2018, Shaker 2020).

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Are steroids and antihistamines really effective for anaphylaxis?: Updated Anaphylaxis Guidelines (2020)


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Post Created By: Akash Bhatnagar, MD (Link)

Peer Review By: Anand Swaminathan MD, MPH

Cite this article as: Akash Bhatnagar MD, "Clinical Conundrums: How Long Should We Monitor After Giving IM Epinephrine for Anaphylaxis?", REBEL EM blog, December 5, 2023. Available at:

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