Which finding best distinguishes a **red thrombus** from a **white thrombus**?

Explanation

## Red Thrombus vs White Thrombus **Key Point:** Red and white thrombi differ primarily in their **cellular composition** — a reflection of the hemodynamic conditions and blood flow rate at the site of formation. ### Composition Comparison | Feature | White (Pale) Thrombus | Red (Dark) Thrombus | |---------|----------------------|--------------------| | **Primary components** | Platelets, fibrin, WBCs | RBCs, fibrin, WBCs | | **Platelet content** | Abundant (60–90%) | Sparse (10–20%) | | **RBC content** | Sparse | Abundant (60–80%) | | **Fibrin content** | Moderate | Abundant | | **Hemoglobin** | Low | High (appears red/dark) | | **Typical location** | Arteries, heart chambers | Veins, distal to thrombus | | **Flow conditions** | High-flow, turbulent | Low-flow, stasis | ### Pathophysiologic Basis **High-Yield:** The composition reflects **hemodynamic conditions**: 1. **White thrombus** forms in **high-flow, turbulent zones** (arteries, cardiac chambers): - Platelets adhere rapidly to endothelium - Fibrin polymerizes around platelet aggregates - RBCs are washed away by fast flow - Result: platelet-rich, fibrin-rich, RBC-poor 2. **Red thrombus** forms in **low-flow, stasis zones** (veins, distal to arterial occlusion): - Coagulation cascade dominates (slow flow allows activation) - Fibrin traps RBCs and WBCs - Platelets are less prominent - Result: RBC-rich, fibrin-rich, platelet-poor ### Lines of Zahn (Laminations) **Clinical Pearl:** Lines of Zahn (alternating layers of platelets and fibrin with RBCs) indicate **antemortem thrombus formation** and are a sign that the thrombus formed *in vivo* with pulsatile flow. They are NOT specific to red or white thrombi — they can occur in both, though more commonly seen in mixed thrombi. **Warning:** Do not confuse "lines of Zahn" with the color distinction. Zahn lines indicate *timing* (antemortem), not composition. ### Mnemonic **WARRED** — **W**hite = Arteries, **A**rteries, **R**apid flow; **R**ed = **R**ed cells, **E**d (venous, stasis), **D**istal. ### Clinical Significance - **White thrombi** → arterial occlusion, MI, stroke, limb ischemia - **Red thrombi** → venous thromboembolism (DVT, PE), post-occlusion tail - **Mixed thrombi** → can occur at the junction of white (proximal) and red (distal) components [cite:Robbins 10e Ch 4]