A 28-year-old man with a history of recurrent sinopulmonary infections and urticaria is found to have low serum C4 levels. Which investigation is most appropriate to confirm the diagnosis of hereditary angioedema (HAE)?

Explanation

## Diagnosis of Hereditary Angioedema (HAE) via C1-INH Assessment ### Clinical Presentation Low C4 with recurrent sinopulmonary infections and urticaria is classic for **hereditary angioedema (HAE)**, which is caused by deficiency or dysfunction of **C1 esterase inhibitor (C1-INH)**. This leads to uncontrolled activation of the classical complement pathway and contact system. ### Why C1-INH Functional Assay is Correct **Key Point:** C1-INH is a serine protease inhibitor that regulates C1q (classical pathway initiator) and kallikrein (contact system). Deficiency or dysfunction of C1-INH causes: - Uncontrolled C1 activation → low C4 consumption - Uncontrolled bradykinin release → angioedema and urticaria **High-Yield:** HAE is diagnosed by **low C4 + abnormal C1-INH (level or function)**. The C1-INH functional assay is the gold standard because it detects both Type I (low level) and Type II (dysfunctional) HAE. ### HAE Classification and Diagnostic Approach ```mermaid flowchart TD A[Angioedema + low C4]:::outcome --> B[Measure C1-INH level and function]:::action B --> C{C1-INH abnormal?}:::decision C -->|Yes| D[HAE Type I or II]:::outcome D --> E{C1-INH level low?}:::decision E -->|Yes| F[Type I HAE<br/>Quantitative deficiency]:::outcome E -->|No| G[Type II HAE<br/>Qualitative deficiency]:::outcome C -->|No| H[Acquired C1-INH deficiency<br/>or other diagnosis]:::outcome ``` ### Diagnostic Tests for HAE | Test | Findings in HAE | Diagnostic Value | |------|-----------------|------------------| | **C4 level** | Low (consumed) | Screening test; not specific | | **C1-INH level** | Low in Type I; normal in Type II | Differentiates Type I vs II | | **C1-INH functional assay** | **Abnormal in both Type I & II** | **Gold standard for diagnosis** | | **C3 level** | Normal | Helps exclude other complement defects | | **AP50** | Normal | Alternative pathway unaffected | **Clinical Pearl:** Type II HAE has **normal or elevated C1-INH levels but reduced function**. Functional assay (not quantitation alone) is essential to diagnose Type II HAE. ### Why This Matters **Key Point:** Distinguishing HAE from other causes of angioedema (allergic, idiopathic) is critical because: - Treatment differs (C1-INH replacement, kallikrein inhibitors, bradykinin antagonists) - Triggers are specific (trauma, stress, ACE inhibitors) - Family screening is needed (autosomal dominant inheritance) [cite:Harrison 21e Ch 313]