Author: Michelle Lin, MD
Updated: 2/24/2012


Definition: A serious allergic reaction that is rapid in onset and might cause death

Mechanism: IgE-mediated immune reaction

Pearl: Hypotension is NOT required to diagnose anaphylaxis.

Triggers: Almost any food, allergens, or medications can be a trigger.

Common culprits:

  • Antibiotics (especially beta-lactams)
  • NSAIDs
  • Peanuts
  • Shellfish

Organ involvement:

  • Skin 80-90%
  • Respiratory 70%
  • Gastrointestinal 45%
  • Cardiovascular 45%
  • Central nervous system 15%

Biphasic anaphylaxis pattern: 2nd flare may occur despite trigger removed (typically within 72 hours of onset)

Diagnostic criteria

Highly likely if 1 or 3 criteria fulfilled per 2nd National Institute of Allergy and ID/Food Allergy and Anaphylaxis Network

  1. Acute onset (min-several hrs), involving skin, mucosa, or both (hives, flushing, pruritis, angioedema) AND at least 1 of the following:
    • Respiratory compromise: shortness of breath, wheezing, stridor, hypoxemia, reduced PEF
    • Reduced BP or associated sx of end-organ dysfunction: hypotonia, syncope, incontinence
  2. Two or more of the following that occur rapidly after exposure to likely allergen:
    • Involvement of skin-mucosal tissue: hives, itching, flushing
    • Respiratory compromise: shortness of breath, wheezing, stridor, hypoxema, reduced PEF
    • Reduced BP or associated dx: hypotonia, syncope, incontinence
    • Persistent GI sx: cramping, abdominal pain, vomiting
  3. Reduced BP after exposure to known allergen
    • Infants/children: Low SBP (age specific) or > 30% decrease in SBP
      • Age 1 mo - 1 yr: SBP < 70 mmHg
      • Age 1 yr - 10 yr: SBP < 70 mmHg + [2 x age]
    • Adults: SBP <90 mmHg or >30% decrease in person's baseline

ED management

  • Supine position, ABCs
  • IM epinephrine STAT. Repeat every 5-15 min if refractory
    • 0.3-0.5 mg for adults = 0.3-0.5 mL 0f 1:1000 concentration of epinephrine
    • 0.15 mg for patient wt < 30 kg
    • IM injection into lateral thigh - quickest absorption centrally
  • IV fluids 2 liters
  • H1 antagonist (eg. diphenhydramine)
  • H2 antagonist (eg. ranitidine)
  • Glucocorticoids (eg. methylprednisolone)
    • 2017 meta-analysis: Unlikely prevents biphasic anaphylactic reaction [Alqurashi]
  • Albuterol (beta-agonist) nebulizer for wheezing/lower airway obstruction
  • Consider: Glucagon 3.5-5 mg IV if refractory to epinephrine and on beta-blockers
  • If discharging patient home after observation, prescribe epinephrine pen


  • Alqurashi W, Ellis AK. Do Corticosteroids Prevent Biphasic Anaphylaxis? J Allergy Clin Immunol Pract. 2017 Sep - Oct;5(5):1194-1205. [Pubmed]
  • Arnold JJ, Williams PM. Anaphylaxis: recognition and management. Am Fam Physician. 2011;84(10):1111-8. [PubMed]
  • Simons FE. Anaphylaxis. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S161-81. [PubMed]