Which laboratory finding best distinguishes Hemophilia A from von Willebrand disease in a patient presenting with mucosal bleeding and a family history of bleeding disorder?

## Distinguishing Hemophilia A from von Willebrand Disease ### Coagulation Profile Comparison | Feature | Hemophilia A | von Willebrand Disease (Type 1/3) | |---------|--------------|------------------------| | **Bleeding time** | Normal | Prolonged (but unreliable/obsolete test) | | **aPTT** | Prolonged | Prolonged (variable) | | **PT** | Normal | Normal | | **Platelet count** | Normal | Normal | | **vWF:Ag** | **Normal** | Reduced | | **Factor VIII activity** | **Reduced** | Reduced (secondary, due to loss of vWF carrier) | | **Bleeding pattern** | Deep tissue, joints, muscles | Mucosal, skin, GI | ### Key Pathophysiologic Difference **Key Point:** The **best laboratory finding** that distinguishes Hemophilia A from von Willebrand disease is **normal vWF antigen (vWF:Ag) with reduced Factor VIII activity** (Option C). In Hemophilia A, the genetic defect is isolated to the *F8* gene encoding Factor VIII — vWF is structurally and quantitatively normal. In von Willebrand disease, the primary defect is in vWF itself, which secondarily reduces Factor VIII levels because vWF acts as a carrier/stabilizer for Factor VIII in plasma. **High-Yield:** Option A (normal bleeding time with prolonged aPTT) is seen in Hemophilia A, but bleeding time is no longer routinely used in modern coagulation workup due to poor standardization and has been largely abandoned since the early 2000s. It is NOT the best distinguishing test per current laboratory practice (Harrison 21e, Williams Hematology 9e). ### Why Option C is the Best Discriminator - **Hemophilia A:** vWF:Ag = Normal; Factor VIII activity = Reduced → isolated Factor VIII deficiency - **vWD (Type 1/3):** vWF:Ag = Reduced; Factor VIII activity = Reduced (secondary) → Option D - **vWD (Type 2N):** vWF:Ag = Normal; Factor VIII activity = Reduced → mimics Hemophilia A but distinguished by vWF:FVIII binding assay Option C specifically identifies the Hemophilia A pattern (normal vWF:Ag + reduced FVIII), which is the hallmark finding that separates it from all vWD subtypes where vWF itself is deficient or dysfunctional. ### Clinical Pearl **Clinical Pearl:** In modern hematology practice, the combination of **vWF:Ag, vWF ristocetin cofactor activity (vWF:RCo), and Factor VIII activity** forms the core panel for distinguishing Hemophilia A from vWD. A normal vWF:Ag with isolated Factor VIII deficiency points directly to Hemophilia A (or rarely vWD Type 2N, which requires further testing). [cite: Harrison 21e Ch 176; Williams Hematology 9e Ch 126]