A 35-year-old man with a history of recurrent angioedema (face, lips, throat) and urticaria presents with an acute episode. His C4 complement level is markedly low (8 mg/dL; normal 15–45). Which investigation is most appropriate to confirm the diagnosis of hereditary angioedema?

## Diagnosis of Hereditary Angioedema (HAE) ### Clinical Presentation The patient presents with: - Recurrent angioedema (face, lips, throat) - Urticaria - **Markedly low C4 level** (8 mg/dL; normal 15–45) This constellation strongly suggests **Type I or Type II Hereditary Angioedema (HAE)**, which is caused by C1-inhibitor (C1-INH) deficiency or dysfunction. ### Key Pathophysiology **High-Yield:** HAE is a Type III hypersensitivity (immune complex-mediated) characterized by: - Deficiency or dysfunction of C1-inhibitor protein - Unopposed activation of contact system (Factor XII, kallikrein) - Excessive bradykinin production → increased vascular permeability - **NOT IgE-mediated** (no urticaria in classic HAE, though this patient has both — suggests overlap or atypical presentation) - **NOT mast cell-dependent** (antihistamines and corticosteroids ineffective) ### Investigation of Choice: C1-Inhibitor Level and Functional Assay **Key Point:** The gold standard for HAE diagnosis is measurement of both **C1-INH antigen level** AND **C1-INH functional activity**. **Clinical Pearl:** Low C4 is a screening clue, but C1-INH testing is confirmatory: - **Type I HAE (85%):** Low C1-INH level + low activity - **Type II HAE (15%):** Normal or elevated C1-INH level but low activity (dysfunctional protein) - **Type III HAE:** Normal C1-INH level and activity; mutations in Factor XII or other contact pathway genes ### Diagnostic Criteria Table | Test | Type I HAE | Type II HAE | Type III HAE | Acquired HAE | |---|---|---|---|---| | **C1-INH Level** | ↓ (< 50% normal) | Normal/↑ | Normal | ↓ | | **C1-INH Activity** | ↓ | ↓ | Normal | ↓ | | **C4 Level** | ↓ | ↓ | Normal | ↓ | | **Genetic Mutation** | SERPING1 | SERPING1 | F12, ANGPT1, KNG1 | Acquired (anti-C1q Ab) | | **Frequency** | 85% | 15% | Rare | Rare | ### Why This Investigation is Superior **C1-INH level + functional assay:** - Directly identifies the molecular defect - Distinguishes Type I (deficiency) from Type II (dysfunction) - Guides therapy (C1-INH replacement vs. other agents) - Highly specific for HAE diagnosis **Rationale for Each Option** **C1-INH level and functional assay (CORRECT):** - Directly measures the defective protein in HAE - Functional assay detects dysfunctional protein (Type II) - Confirms diagnosis and determines HAE type - Essential for family screening and genetic counseling **Skin prick test:** - Detects IgE-mediated (Type I) hypersensitivity - HAE is NOT IgE-mediated; SPT will be negative - Does not explain low C4 or C1-INH deficiency - Irrelevant to contact system activation **IgE level and specific IgE:** - Measures humoral immune response to allergens - HAE is complement-mediated, not IgE-mediated - Will not identify C1-INH deficiency - Does not explain angioedema mechanism **Histamine release assay:** - Detects mast cell degranulation (Type I hypersensitivity) - HAE is NOT mast cell-dependent - Antihistamines are ineffective in HAE - Does not measure C1-INH or contact system activation ### Mnemonic: **C1-INH DEFECT** - **C1-INH** = C1-inhibitor protein - **DEFECT** = Deficiency (Type I) or Dysfunction (Type II) - **E** = Essential to measure both level AND activity - **C** = Confirms diagnosis; guides therapy - **T** = Type I vs. Type II distinction ### Clinical Correlation **Warning:** Do NOT confuse: - ~~Allergic angioedema~~ (IgE-mediated, urticaria present, responds to antihistamines) — HAE is NOT allergic - ~~ACE inhibitor-induced angioedema~~ (drug-induced, acquired C1-INH deficiency) — check medication history - ~~Acquired angioedema~~ (anti-C1q antibodies, lymphoproliferative disease) — C1-INH level is low but often with lymphoma [cite:Harrison 21e Ch 297; Robbins 10e Ch 6]