Results of our study show similar sensitivity of the fC1INH LC-MS/MS assay from DBS to identify patients with recurrent angioedema due to C1INH deficiency when contrasted with the fC1INH chromogenic assay, and similar specificity of all 3 fC1INH assays. The gene discussed is SERPING1; the disease is hyperinsulinemic hypoglycemia, familial, 4.