The post-anaphylaxis dilemma: how long is long enough to observe a patient after resolution of symptoms?

Anaphylaxis may recur 1 to 72 hours after initial signs and symptoms apparently resolve. Reported incidence of biphasic anaphylaxis varies from 1% to 23%. Late-phase severity varies from mild to severe (rarely fatal). No evidence or expert consensus presently identifies distinguishing characteristics in the initial phase that predict a late phase. However, reports propose potential risk factors, including initial phase severity, delayed or suboptimal doses of epinephrine during initial treatment, laryngeal edema or hypotension during initial phase, delayed symptomatic onset after antigen exposure, or history of biphasic anaphylaxis. It is unclear whether systemic corticosteroids administered in the initial phase can prevent or weaken late-phase reactions. Based on available evidence, observation periods after complete resolution of uniphasic anaphylaxis should be individualized, particularly because there are no reliable predictors of biphasic anaphylaxis. A 10-hour observation period appears sufficient, but some investigators recommend 24 hours.