_ Definition:Biphasic anaphylaxis is the recurrence of symptoms within 72 hours with no further exposure to the allergen. It occurs in between 1–20% of cases depending on the study examined. It is managed in the same manner as anaphylaxis

The incidence of biphasic anaphylactic reactions as described in the literature is highly variable, ranging from a low of 1% to a high of 20% of episodes. There is no clear consensus regarding distinguishing features of the primary response that predicts the occurrence of a secondary response. However, the severity of the reaction, the time of onset after administration of antigen before the occurrence of symptoms of the primary response, the presence of hypotension or laryngeal edema during the primary response, and the history of a previous biphasic reaction have all been mentioned as risk factors in various studies. The severity of the late-phase reaction is highly variable, and events have ranged from mild to severe with rare fatalities. Most late-phase reactions, however, are mild to moderate in severity. A delay in the administration of epinephrine and too small a dose of epinephrine given for the primary response have also been mentioned as risk factors. It is unclear whether corticosteroids given for the primary event can prevent or ameliorate the second reaction.

_Conclusions: Biphasic responses occur with significant frequency and therefore should be taken into consideration when one considers the observation period after the initial event. An observation period of 8 hours is sufficient for most reactions, but since reactions can occur as long as 72 hours after resolution of the primary event, some authors have recommended a 24-hour waiting period.
http://en.wikipedia.org/wiki/Anaphylaxis
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