## Distinguishing FES from ARDS **Key Point:** Petechial rash is the pathognomonic clinical sign of fat embolism syndrome and is absent in ARDS. It typically appears 24–72 hours after injury on the trunk, axillae, conjunctivae, and oral mucosa. ### Comparison Table | Feature | FES | ARDS | | --- | --- | --- | | **Petechial rash** | Present (24–72 hrs) | Absent | | **Chest X-ray** | Bilateral infiltrates ("snow storm") | Bilateral infiltrates | | **Hypoxemia** | Yes, refractory | Yes, refractory | | **Thrombocytopenia** | Common (diagnostic) | May occur | | **Elevated lipase** | Yes (serum/urine) | No | | **Trigger** | Long bone fractures, trauma | Sepsis, aspiration, transfusion | **High-Yield:** The **Gurd criteria** for FES diagnosis include petechial rash as a major criterion. Its presence strongly suggests FES over other causes of post-traumatic respiratory failure. **Clinical Pearl:** FES typically manifests 24–48 hours after long bone fracture; early respiratory symptoms (within 12 hours) are more suggestive of aspiration or ARDS from other causes. ### Why Other Features Are Non-Discriminatory - **Bilateral infiltrates** and **hypoxemia** occur in both conditions—they represent the common final pathway of acute lung injury. - **Elevated inflammatory markers** are non-specific and present in both FES and ARDS. - The discriminator is the **petechial rash**, which is specific to FES and results from microinfarction due to fat globule lodgment in dermal capillaries. [cite:Rockwood & Green's Fractures in Adults Ch 1] 
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