BACKGROUND: Hereditary angioedema types I and II are caused by a functional deficiency of C1 inhibitor (C1-INH) leading to overproduction of bradykinin. The current functional diagnostic assays employ inhibition of activated C1s, however, an alternative, more physiologic method, is desirable. METHODS: ELISA assays were developed using biotinylated activated factor XII (factor XIIa) or biotinylated kallikrein bound to avidin-coated plates. Incubation with plasma was followed by detection of bound C1-INH. RESULTS: After standard curves were developed for quantification of C1-INH, serial dilutions of normal plasma were employed to validate the ability to detect known concentration of C1-INH in the plasma as a percent of normal. HAE types I and II were then tested. The level of functional C1-INH in all HAE types I and II plasma tested were less than 40% of our normal control. This was evident regardless whether we measured factor XIIa-C1-INH or kallikrein-C1-INH complexes, and the two assays were in close agreement. By contrast, testing the same samples utilizing the commercial method (complex ELISA, Quidel Corp.) revealed levels of C1-INH between 0 and 57% of normal (mean, 38%) and 42 samples were considered equivocal (4 controls and 38 patients). CONCLUSIONS: Diagnosis of HAE types I and II can be ascertained by inhibition of enzymes of the bradykinin-forming cascade; namely, factor XIIa and kallikrein. Either method yields functional C1-INH levels in HAE patients (types I & II) that are clearly abnormal with less variance or uncertainty than the commercial method. This article is protected by copyright. All rights reserved.