Written by Clay Smith
Out of 83 patients with severe anaphylaxis, 1 had a probable biphasic allergic reaction, which was mild and consisted of rash only. No patients developed true biphasic anaphylaxis.
Why does this matter?
Ah, the infamous biphasic reaction. Is this real? The literature reports biphasic allergic reaction occurs anywhere from 1-23% of the time. This study looked at severe cases of anaphylaxis in the ICU to see if they could find answers.
Is there such a thing as biphasic anaphylaxis?
This was a retrospective review of 83 patients at a single center with severe anaphylaxis, requiring ICU admission. Of these, 99% received antihistamines, 90% steroids, and just 80% epinephrine. One patient out of 83 (1.2%) had probable biphasic allergic reaction, and this was merely skin changes, not life-threatening and not anaphylactic, according to the study definition. Of the 3 possible and 1 probable biphasic allergic reactions, they occurred at an average of 14 hours from the initial reaction. This study had some issues. The most important is that of patients with severe anaphylaxis, 20% didn’t receive the most important therapy - epinephrine. So, even with suboptimal treatment, the incidence of biphasic allergic reaction within 72 hours was very low and the single probable reaction was mild. Another issue is the retrospective nature was limited in determining what was and was not an allergic reaction, such as rash or hypotension for other reason, such as sepsis. My take home point is that true biphasic anaphylactic reaction is rare or non-existent. Most patients simply need to be observed a few hours to ensure they don’t have persistent anaphylaxis, especially those with ingestion of a food as the cause of the reaction.
The Skeptics Guide to EM did an in-depth, outstanding summary of biphasic anaphylaxis. Their bottom line: “Prolonged observation is likely unnecessary,” once symptoms resolve in the ED. Don’t miss this post.
Low Incidence of Biphasic Allergic Reactions in Patients Admitted to Intensive Care after Anaphylaxis. Anesthesiology. 2018 Nov 5. doi: 10.1097/ALN.0000000000002500. [Epub ahead of print]
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