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Post-It Pearls: Observing Patients With Biphasic Anaphylaxis: Emergency Medicine News


biphasic anaphylaxis, epinephrine


How long do you observe emergency department patients with anaphylaxis for a recurrent episode (biphasic anaphylaxis)? This phenomenon occurs in about five percent of patients and can occur at any time during the first 72 hours.

The answer is not clear, but experts generally agree on the higher risk predictors below, which are based on a meta-analysis reporting the recommendations of the Joint Task Force on Practice Parameters 2020. (J Allergy Clin Immunol. 2020; 145[4]: 1082.)

The patient should be given an extended observation period and should be admitted for careful monitoring if any of the following are present:

  • Severe initial presentation (such as hypotension or an increase in differential pressure> 100 mm Hg) – OR 2.11 (95% CI: 1.23-3.61)
  • More than one dose of epinephrine required on initial presentation – OR 4.82 (95% CI, 2.70-8.58)
  • UpToDate also suggests including patients who “only received epinephrine after a long delay (more than 60 minutes)”.

The literature is not as strong, but also consider these risk factors possibly associated with biphasic anaphylaxis:

  • Drug trigger in children – OR 2.35 (95% CI, 1.16-4.76)
  • Unknown trigger of anaphylaxis – OR 1.63 (95% CI, 1.14-2.33)

Dr Linis the Founder, CEO and Editor-in-Chief of Academic Life in Emergency Medicine ( and professor of emergency medicine at the University of California at San Francisco with interests in teaching the health professions and digital scholarship. Follow her on Instagram@MichelleLinMDand on Twitter@M_Lin.