Incidence and characteristics of biphasic anaphylaxis: a prospective evaluation of 103 patients

Background: Although it is known that anaphylaxis can follow a biphasic course, reports of its incidence are conflicting. Furthermore, little is known about predictors of biphasic reactivity. Objective: To describe the incidence and characteristics of biphasic anaphylaxis occurring in a Canadian tertiary care center. Methods: All patients with emergency department visits and inpatients given a diagnosis of "allergic reaction" or "anaphylaxis" during a 3-year period were evaluated. Patients were contacted within 72 hours to establish symptoms and determine the presence of biphasic reactivity. A full medical record review ensued, and uniphasic and biphasic cases were compared using the Mann-Whitney U test for continuous data and the chi(2) and Fisher exact tests for ordinal data. Results: A total of 134 patients with anaphylaxis were identified; complete follow-up was obtained for 103 patients. Twenty patients (19.4%) experienced confirmed biphasic reactivity. Average time to onset of the second phase was 10 hours (range, 2-38 hours); 8 patients (40.0%) had their second phase occur more than 10 hours after the initial reaction. The clinical presentations and histories of uniphasic and biphasic reactors were similar. Time to resolution of initial symptoms was significantly longer for biphasic reactors (112 vs 133 minutes; P =.03). Differences in management were noted: biphasic reactors received less epinephrine (P =.048) and tended to receive less corticosteroid (P =.06). Conclusions: Biphasic reactivity occurred with an incidence of 19.4%, consistent with first descriptions. The second-phase onset was 10 hours on average, but it occurred as late as 38 hours. Biphasic anaphylaxis may be related, in part, to undertreatment.