• IV fluids
Biphasic anaphylaxis can typically occur anytime within 4 days, but is reported to occur in only about 10% of patients. However, clinically significant biphasic reaction is felt to occur in around 1% of patients.
SO... the question is how long should a patient be medically observed for biphasic anaphylaxis after successfully treatment of the initial anaphylactic reaction?
There is no consensus on what the ideal duration of observation should be after successful treatment of anaphylaxis, but most treating physicians (emergency room, allergists, and ENTs) would suggest 1-2 hours for mild anaphylactic reactions and up to 4 hours for moderately severe anaphylactic reactions.
Observation in the hospital for 24 hours would be reserved for patients who suffered from severe anaphylaxis with the following history:
• Delayed administration of epipen or required more than 1 dose of epinephrine
• Unknown trigger
• Required multiple IV fluid boluses to treat significant hypotension
With discharge from medical observation, patients are usually instructed to do the following (given the low risk for possible anaphylactic recurrence for up to 4 days):
• Take benadryl every 6 hours and/or zyrtec twice a day for at least 4 days
• Take a course of steroids (I usually prescribe a medrol dose-pack)
• Have an epipen immediately available on hand for at least 4 days
• If symptoms DO recur, administer epipen and go back to the ER
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Time of Onset and Predictors of Biphasic Anaphylactic Reactions: A Systematic Review and Meta-analysis. J Allergy Clin Immunol Pract. 2015 May-Jun;3(3):408-16.e1-2. doi: 10.1016/j.jaip.2014.12.010. Epub 2015 Feb 11.
Update on biphasic anaphylaxis. Curr Opin Allergy Clin Immunol. 2016 Aug;16(4):346-51. doi: 10.1097/ACI.0000000000000279.
Biphasic anaphylaxis: review of incidence, clinical predictors, and observation recommendations. Immunol Allergy Clin North Am. 2007 May;27(2):309-26, viii.
Incidence of clinically important biphasic reactions in emergency department patients with allergic reactions or anaphylaxis. Ann Emerg Med. 2014 Jun;63(6):736-44.e2. doi: 10.1016/j.annemergmed.2013.10.017. Epub 2013 Nov 13. 3>