A 38-year-old woman with acute promyelocytic leukemia (APL) presents with fever, bleeding, and a platelet count of 15,000/μL. Laboratory studies show PT 18 s (control 12 s), aPTT 52 s (control 35 s), fibrinogen 80 mg/dL, and D-dimer >10 μg/mL. Which finding best distinguishes DIC secondary to APL from primary fibrinolysis?

Explanation

## DIC vs. Primary Fibrinolysis in APL ### Pathophysiologic Context **Key Point:** APL (acute promyelocytic leukemia) triggers DIC via release of tissue factor and cancer procoagulants from abnormal promyelocytes, leading to systemic thrombin generation and consumption of platelets and clotting factors. Primary fibrinolysis, by contrast, involves excessive plasmin activity that degrades fibrinogen and fibrin but does **not** significantly consume platelets or clotting factors. ### Comparison Table: DIC vs. Primary Fibrinolysis | Feature | DIC (APL) | Primary Fibrinolysis | |---------|-----------|---------------------| | **Mechanism** | Thrombin generation → fibrin deposition → consumption of platelets + factors | Excessive plasmin → fibrinogen/fibrin breakdown without thrombin activation | | **Platelet count** | **Low** (consumption) | **Normal or near-normal** | | **PT/aPTT** | Prolonged | Normal or mildly prolonged | | **Fibrinogen** | Low (consumption) | Low (degradation) | | **Factor V** | **Low** (consumption) | **Normal or elevated** | | **Factor VIII** | **Low** (consumption) | **Normal or elevated** | | **D-dimer** | Markedly elevated | Elevated (but may be lower) | | **FDPs** | Elevated | Markedly elevated | | **Response to heparin** | May improve | Worsens | ### High-Yield Discriminator **High-Yield:** The **best distinguishing feature** of DIC from primary fibrinolysis is **severe thrombocytopenia combined with depressed Factor V and Factor VIII levels** (Option C). - In **DIC**: Thrombin generation causes platelet aggregation and consumption → thrombocytopenia; simultaneously, factors V and VIII are consumed in the coagulation cascade → low levels. - In **primary fibrinolysis**: Plasmin degrades fibrinogen and fibrin, but platelets are **not consumed** (count remains normal or near-normal), and factors V and VIII are **not consumed** (levels remain normal or elevated). Option A (elevated D-dimer + low fibrinogen + prolonged PT/aPTT) describes findings seen in **both** DIC and severe primary fibrinolysis — it does **not** best distinguish between the two. Option C is the only finding that is characteristic of DIC but **absent** in primary fibrinolysis, making it the best discriminator. ### Clinical Pearl **Clinical Pearl:** Per Harrison's Principles of Internal Medicine, the key laboratory distinction between DIC and primary fibrinolysis is the **platelet count and factor V/VIII levels**: DIC consumes both platelets and coagulation factors, whereas primary fibrinolysis spares them. This distinction has direct therapeutic implications — antifibrinolytic agents (e.g., tranexamic acid) may be used in primary fibrinolysis but are contraindicated in DIC. ### Mnemonic **Mnemonic:** - **DIC = Consumption** (platelets ↓↓, Factor V ↓, Factor VIII ↓, fibrinogen ↓, D-dimer ↑↑) - **Primary Fibrinolysis = Degradation only** (platelets normal, Factor V normal, Factor VIII normal, fibrinogen ↓, FDPs ↑↑↑)