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Recurrent manifestations of anaphylaxis after treatment, termed biphasic reactions, are estimated to occur in 1 to 23% of reactionsHowever, little is known about the incidence, triggers, symptoms, and management of biphasic reactionsIn 83 cases of patients admitted to intensive care units in Denmark after anaphylaxis, suspected biphasic reactions occurred in 4 (4.8%) of patientsThe incidence of biphasic reactions was low, 3 out of 4 were considered possible, and only 1 considered a probable biphasic allergic reactionBiphasic allergic reactions—recurrence of allergy symptoms after a symptom-free period—are reported to occur in 1 to 23% of allergic reactions. Patients admitted to an intensive care unit after anaphylaxis potentially have more severe reactions and a higher risk of biphasic allergic reactions. The purpose of this study was to examine incidence, triggers, symptoms, and treatment of biphasic allergic reactions, in patients admitted to an intensive care unit.Records of patients admitted to intensive care units with anaphylaxis from 2011 to 2014 were reviewed. Only patients with a reaction fulfilling internationally accepted criteria for anaphylaxis were included. Potential biphasic allergic reactions, defined as renewed allergy symptoms 1 to 72 h after initial symptoms had resolved, without further exposure to the trigger, were identified.A total of 83 cases of anaphylaxis were identified, and the most frequent triggers were medications (58 of 83 [70%]). Skin symptoms occurred in 69 (83%) cases, and circulatory and respiratory symptoms in 48 (58%) and 45 (54%) cases, respectively. In total, 82 (99%), 80 (96%), and 66 (80%) were treated with antihistamines, corticosteroids, and epinephrine, respectively. Only 10 patients presented with one or more relevant symptoms after the initial allergic reaction. Of these, three were possible, and one was a probable biphasic allergic reaction, giving a total incidence of 4 of 83 (4.8% [95% CI, 1.6 to 12.5]) or 1 of 83 (1.2% [95% CI, 0.1 to 7.46]), respectively. All cases were mild, presenting with skin symptoms only, occurring on average 14 h after initial reactions.The authors observed a low incidence of biphasic reactions in patients admitted to an intensive care unit after anaphylaxis, at a rate equivalent to that reported in other patient groups.