The 2021 World Allergy Organization/European Academy of Allergy and Clinical Immunology guidelines recommended the following for differential diagnosis of HAE-nC1INH: suspicion of HAE should prompt laboratory testing to assess C1INH function, C1INH protein levels, and C4 levels; and patients who are suspected to have HAE and have normal C1INH levels and function should undergo genetic testing for known mutations underlying HAE-nC1INH.2 Here, C4A is linked to hereditary angioedema.