function and antigenic C4 level) and contact phase activation (via amidase activity). Bradykinin is respon-sible for angioedema attacks and is produced from contact phase activation secondary to failed C1Inhcontrol.Objective: We aimed to compare the diagnostic performances of spontaneous amidase activity and anti-genic C4 level in C1Inh hereditary angioedema (C1Inh-HAE) patients.Methods: Samples from 185 C1Inh-HAE patients (81 men, 104 women; confirmed by SERPING1 genemutations) and from 99 blood donors (50 men, 49 women) were tested for C1Inh function, antigenic C4level and spontaneous amidase activity.Results: In the C1Inh-HAE group, antigenic C4 level was decreased (n = 135) and amidase activity wasincreased (n = 181). Receiver operating characteristic analyses showed higher diagnostic performancevalues for the spontaneous amidase assay compared to those of antigenic C4.Conclusion: The spontaneous amidase activity assay should replace antigenic C4 level testing and shouldbe tested alongside the C1Inh function for both AE screening and follow up of HAE patients.