BACKGROUND : Biphasic reaction is the recurrence of anaphylaxis symptoms within 72 hours of the initial anaphylactic event, without re-exposure to the trigger. Biphasic reactions caused by antibiotics are uncommon, unpredictable and their risk factors are poorly understood and studied in the literature. 

OBJECTIVE : To identify the predictors of biphasic reactions in patients with anaphylaxis to antibiotics in the emergency department (ED). 

METHODS: It was a prospective and monocentric study over !ve years (2010 to 2015). Adult patients who presented to the ED and met diagnostic criteria for anaphylaxis to antibiotics  were included. Epidemiological, clinical and therapeutic characteristics for all patients were collected. A multivariate analysis of predictors of biphasic reactions to antibiotics was conducted. 

RESULTS : One hundred and twenty !ve patients were enrolled. "e average age was 42 ± 14 years. Sex ratio = 0.95. Anaphylactic history was reported in 41 % of patients. The most common antibiotic family was Penicillin in 82 % of cases. Biphasic anaphylactic reactions were observed in 5%. Parenteral administration of antibiotics(adjusted OR = 3.38, 95% CI [1.39 - 7.99], p = 0.02), respiratory signs (adjusted OR = 2.25; 95% CI [1.45 - 18.35], p = 0.03) and the use of epinephrine 100 μg (adjusted OR = 3.84, 95% CI [1.52- 4.31]; p<0.001) were independently associated with biphasic reaction. The improvement was noted in 123 patients (99%). No deaths were recorded. Nighty-nine percent of patients were discharged directly from ED after a mean period of observation at six hours. 

CONCLUSION : The biphasic reactions of anaphylaxis to antibiotics remain under diagnosed in the ED. The parenteral administration of antibiotics, the respiratory signs and the use of adrenaline 100 μg were associated with a high risk of recurrence of anaphylaxis in the ED.