## Distinguishing Factor V from Factor VIII Deficiency ### The Critical Distinction: Thrombin Time **Key Point:** Factor V is a cofactor in both the intrinsic (aPTT) and extrinsic (PT) pathways, as well as in the final common pathway (thrombin generation). Factor VIII participates only in the intrinsic pathway. A prolonged thrombin time (TT) indicates involvement of the final common pathway, which occurs in Factor V deficiency but not Factor VIII deficiency. ### Comparative Table | Coagulation Test | Factor V Deficiency | Factor VIII Deficiency | |------------------|-------------------|------------------------| | **PT** | Prolonged | Normal | | **aPTT** | Prolonged | Prolonged | | **Thrombin Time (TT)** | Prolonged | Normal | | **Platelet count** | Normal | Normal | | **Bleeding time** | Normal | Normal | | **Mixing study** | Corrects aPTT | Corrects aPTT | | **Fibrinogen** | Normal | Normal | ### Pathophysiologic Basis **Factor V Role:** - Essential cofactor in prothrombinase complex (Factor Xa + Factor V + phospholipid + Ca²⁺ → thrombin) - Participates in both extrinsic AND intrinsic pathways - Required for thrombin generation from prothrombin **Factor VIII Role:** - Cofactor only in the intrinsic tenase complex (Factor IXa + Factor VIII + phospholipid + Ca²⁺ → Factor Xa) - Does NOT participate in the extrinsic pathway or final common pathway - Deficiency does NOT prolong PT or TT **High-Yield:** The **prolonged PT in Factor V deficiency** is the most obvious discriminator, but since the stem specifies "normal PT," the next-best discriminator is **prolonged TT**, which reflects Factor V's role in the final common pathway. **Mnemonic:** **"V is for Very versatile"** — Factor V participates in all three major pathways (intrinsic, extrinsic, common). Factor VIII is **"VIII = Intrinsic only"**.
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