Which finding best distinguishes Hemophilia A from von Willebrand disease in a patient presenting with mucosal bleeding and a prolonged aPTT?

Explanation

## Distinguishing Hemophilia A from von Willebrand Disease ### Key Discriminator: Platelet Function Testing **Key Point:** Abnormal ristocetin-induced platelet aggregation (RIPA) is the hallmark finding that distinguishes von Willebrand disease from Hemophilia A. This test directly assesses von Willebrand factor's role in platelet adhesion. ### Comparative Table | Feature | Hemophilia A | von Willebrand Disease | |---------|--------------|------------------------| | **aPTT** | Prolonged | Prolonged (variable) | | **Bleeding time** | Normal | Abnormal/prolonged | | **Platelet count** | Normal | Normal | | **RIPA** | Normal | Abnormal (↓ or absent) | | **Factor VIII** | Markedly ↓ | Normal or mildly ↓ | | **vWF antigen** | Normal | Reduced | | **vWF activity** | Normal | Reduced | | **Inheritance** | X-linked recessive | Autosomal dominant (Type 1) | ### Why RIPA Is the Discriminator 1. **Mechanism:** Ristocetin is an antibiotic that causes platelet agglutination only in the presence of functional von Willebrand factor. In vWD, deficient or dysfunctional vWF prevents this platelet response. 2. **In Hemophilia A:** Platelet function is completely normal because the defect is in factor VIII (a coagulation factor), not in platelet adhesion or vWF. 3. **Clinical Significance:** While both present with prolonged aPTT and mucosal bleeding, the platelet function defect in vWD creates an additional bleeding diathesis affecting primary hemostasis. **High-Yield:** RIPA is the gold-standard confirmatory test for von Willebrand disease and is always normal in Hemophilia A. **Clinical Pearl:** Patients with vWD often have abnormal bleeding time (reflecting platelet dysfunction), whereas Hemophilia A patients have normal bleeding time despite severe bleeding tendency — a seemingly paradoxical finding that hinges on the distinction between primary and secondary hemostasis.