## Distinguishing Thrombus from Embolus **Key Point:** The fundamental difference between a thrombus and an embolus lies in their **site of formation and origin**, not their composition or consequences. ### Definition and Origin | Feature | Thrombus | Embolus | |---------|----------|----------| | **Site of formation** | Forms *in situ* within the vessel lumen | Forms elsewhere and travels to lodge distally | | **Origin** | Originates from vessel wall or blood stasis | Detaches from a distant source (heart, atherosclerotic plaque, paradoxical shunt) | | **Attachment** | Remains attached to vessel wall | Travels freely through circulation | | **Onset** | Gradual formation over time | Sudden arrival and impaction | ### Composition Similarity **High-Yield:** Both thrombi and emboli have **identical composition** — fibrin, platelets, RBCs, WBCs, and coagulation factors. This is why composition alone cannot distinguish them. ### Clinical Consequences Both can cause: - Sudden vascular occlusion - Tissue ischemia and infarction - Inflammatory response - Potential recanalization **Clinical Pearl:** An embolus lodging in a vessel will appear histologically identical to a thrombus once it occludes the vessel — the distinction is **temporal and anatomical**, not morphological. ### Why Origin Matters **Mnemonic:** **SITE** — **S**ite of formation, **I**n situ vs traveling, **T**hrombus local vs **E**mbolus external origin. Identifying the source of an embolus (cardiac, arterial, venous paradoxical) is critical for: 1. Determining underlying pathology 2. Guiding prevention strategies (anticoagulation, antiplatelet therapy, source control) 3. Predicting recurrence risk [cite:Robbins 10e Ch 4] 
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