In a patient with hemophilia A, which of the following findings would be expected on coagulation studies?

Explanation

## Hemophilia A: Coagulation Profile **Key Point:** Hemophilia A is a deficiency of Factor VIII (intrinsic pathway). The coagulation profile reflects selective impairment of the intrinsic pathway only. ### Factor VIII and the Intrinsic Pathway Factor VIII is a cofactor in the intrinsic pathway complex: - **Intrinsic pathway:** Factors XII → XI → IX → VIII (with IXa, VIIIa, Ca²⁺, phospholipid) - **Extrinsic pathway:** Tissue Factor + Factor VII (independent of Factor VIII) - **Common pathway:** Factors X → V → II → Fibrinogen (independent of Factor VIII) ### Expected Coagulation Studies in Hemophilia A | Test | Result | Reason | |------|--------|--------| | **PT** | Normal | Extrinsic pathway (TF + VII) is intact | | **aPTT** | Prolonged | Intrinsic pathway (Factor VIII deficiency) | | **Bleeding time** | Normal | Platelet function and primary hemostasis intact | | **Thrombin time** | Normal | Fibrinogen and final common pathway intact | | **Platelet count** | Normal | Thrombocytopoiesis unaffected | **High-Yield:** The **aPTT corrects with normal plasma** (bypassing the Factor VIII deficiency by adding exogenous Factor VIII from donor plasma) — this is the hallmark diagnostic feature. **Clinical Pearl:** Hemophilia A severity correlates with Factor VIII level: - **Severe:** <1% activity → spontaneous bleeding, hemarthrosis - **Moderate:** 1–5% → bleeding with trauma - **Mild:** 5–40% → bleeding with major trauma or surgery **Mnemonic:** **FVIII = Intrinsic** — Factor VIII deficiency → Intrinsic pathway prolongation → aPTT prolonged. [cite:Harrison 21e Ch 139]