sgp120 and the contact system in hereditary angioedema: A diagnostic tool in HAE with normal C1 inhibitor.

•sgp120 has been identified as ITIH4, can be cleaved by kallikrein, and can be used to study contact system activation.•sgp120 fragmentation and spontaneous contact system activation is present in the plasma of a majority of HAE-C1inh patients.•sgp120 fragmentation and contact system activation occur in a majority of HAE-nl-C1inh samples after cold incubation.•The loss of C1 inhibitor function and C1 inhibitor fragmentation occur in HAE-nl-C1inh samples after cold incubation.•sgp120 (ITIH4) may be a potential biomarker for the laboratory diagnosis of HAE-nl-C1inh.