Abstract

While it has been recognized for over a quarter century that anaphylactic reactions have the potential to follow a biphasic course, reports on the incidence of biphasic anaphylaxis are conflicting, and the search for reliable predictive factors of such responses has been challenging. Further adding to the complexity of this clinical entity are the widely variable durations of the asymptomatic window, and the similarly variable reports on second phase severity. This review aims to provide the health care professional with a better understanding of the true incidence, nature, and risk factors for this type of reactivity by consolidating and summarizing the available literature on the topic of biphasic anaphylaxis. As our body of evidence builds, patterns are emerging to suggest that those patients with an initial presentation requiring more than one dose of epinephrine, those who have life-threatening initial presenting features, and those who otherwise take longer to stabilize, are in this higher risk group, and would be more likely to benefit from prolonged in hospital observation. Conversely, patients who respond rapidly to the immediate administration of epinephrine may be at lower risk, but this finding requires confirmation by others. Further prospective evaluations of biphasic anaphylaxis will greatly aid our understanding of this condition.