Podcast 994: Biphasic Anaphylaxis

Biphasic reactions in anaphylaxis present a clinical challenge regarding the appropriate observation period for patients after their initial symptoms resolve. While historical estimates for these secondary reactions varied widely from 1% to 20%, a recent study indicates they occur in approximately 16% of cases. Crucially, these recurrences typically manifest much later than previously thought, with an average onset of 19 hours and rarely occurring before the eight-hour mark. These secondary episodes are generally milder than the initial reaction and often do not require additional epinephrine. Because most patients can be safely discharged after a standard two-to-four-hour observation window, the focus of care shifts to patient education. Effective management relies on ensuring patients receive an epinephrine auto-injector and understand that symptoms may return up to 24 hours later, requiring prompt self-treatment or a return to the emergency department.