The objectives of this study were to: 1) determine the incidence of biphasic reactions in children with anaphylaxis; 2) establish what risk factors can predict progression to a biphasic reaction; and 3) assess the utility of inpatient observation for patients whose anaphylaxis appears to have resolved.


We performed a retrospective analysis of all children admitted to Children's Hospital inpatient service between 1985 and 1999 with acute anaphylaxis. Data were collected from the medical records regarding past medical history, presenting signs and symptoms, treatment, and hospital course. Patients were considered to have resolution of anaphylaxis if they were documented to have cessation of all symptoms and needed no therapy for at least 1 hour. Biphasic reactions were defined as a worsening of symptoms requiring new therapy after resolution of anaphylaxis. Significant biphasic reactions were defined as those requiring oxygen, vasopressors, intubation, subcutaneous epinephrine, or unscheduled bronchodilator treatments. Patients were considered to benefit from a 24-hour observation period if they had a significant biphasic reaction within 24 hours of admission.


Of 108 anaphylactic episodes, 2 (2%) were fatal, and 1 (1%) was a protracted anaphylactic reaction. Among the remaining 105 children with resolution of anaphylaxis, 6 (6%) [95% confidence (CI): 2, 12] had biphasic reactions, of which 3 (3%) [95% CI: .6, 8] were significant. Of those who had a biphasic reactions, the median time from the onset of symptoms to the initial administration of subcutaneous epinephrine was 190 minutes, versus 48 minutes for those without a biphasic reaction. Patients with or without biphasic reactions did not differ significantly in the incidence of initial epinephrine use, initial steroid use, or serious respiratory or cardiovascular symptoms on initial presentation. Two of 105 (2%) [95% CI: .2, 7] patients clinically benefitted from a 24-hour observation period.


We found an overall incidence of biphasic reactions of 6%, and an incidence of significant biphasic reactions of 3%, among pediatric patients admitted with anaphylaxis. Delayed administration of subcutaneous epinephrine was associated with an increased incidence of biphasic reactions. Approximately 2% of patients with anaphylaxis potentially benefitted from a 24-hour period of observation after symptoms had resolved.

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