Which laboratory finding best distinguishes DIC from primary fibrinolysis in a 38-year-old woman with sepsis-induced coagulopathy?

Explanation

## Distinguishing DIC from Primary Fibrinolysis ### Key Pathophysiologic Difference **Key Point:** DIC is a **consumption coagulopathy** (thrombin generation → fibrin deposition → consumption of platelets, fibrinogen, and clotting factors), whereas primary fibrinolysis is **fibrin breakdown without prior thrombosis** (excess plasmin activity). ### Laboratory Comparison Table | Parameter | DIC | Primary Fibrinolysis | | --- | --- | --- | | **Fibrinogen** | ↓↓ (consumed) | ↓ (degraded) or normal | | **Platelets** | ↓↓ (consumed in microthrombi) | Normal or mildly ↓ | | **D-dimer** | ↑↑↑ (massive) | ↑ (elevated) | | **FDP** | ↑↑↑ | ↑↑↑ | | **PT/INR** | ↑ (factor consumption) | Normal or mildly ↑ | | **Thrombin time** | ↑ (low fibrinogen + FDP) | ↑ (FDP interference) | | **Prothrombin fragment 1+2** | ↑↑↑ (thrombin generation) | Normal | | **Fibrin monomer** | ↑↑↑ | Normal | ### Why Option 0 Is Correct **High-Yield:** In **primary fibrinolysis**, the fibrinolytic system is activated *without* preceding widespread thrombosis. Plasmin directly degrades circulating fibrinogen and fibrin, causing: - **Markedly elevated D-dimer** (from fibrin breakdown) - **Normal or near-normal fibrinogen** (because there was no prior consumption; fibrinogen is being degraded *in situ* but not depleted to critical levels) - Platelet count typically **normal** (no consumption in microthrombi) In contrast, **DIC** shows: - Elevated D-dimer (from microthrombi formation and lysis) - **Severely low fibrinogen** (consumed during thrombin-driven fibrin formation) - **Thrombocytopenia** (consumed in microthrombi) **Clinical Pearl:** The **fibrinogen level is the most discriminating single test**. A patient with a markedly elevated D-dimer but *preserved* fibrinogen (>100 mg/dL) strongly suggests primary fibrinolysis; a low fibrinogen (<100 mg/dL) with elevated D-dimer and thrombocytopenia is classic DIC. ### Mnemonic **FIBRINOLYSIS = Fibrin degradation without consumption** — fibrinogen stays relatively intact because it was never consumed in a thrombotic process. **DIC = Dual injury** — both thrombosis (consumption) and secondary fibrinolysis (breakdown).