A 52-year-old man with known cirrhosis presents with dyspnea. Chest X-ray shows a moderate right-sided pleural effusion. On ultrasound, the effusion measures 3 cm in depth at the posterior costophrenic angle. Which imaging feature best distinguishes a transudative pleural effusion from an exudative one?

## Distinguishing Transudative from Exudative Effusions on Imaging ### Ultrasound Findings **Key Point:** Transudative effusions typically appear anechoic (simple fluid), while exudative effusions often contain internal echoes, fibrin strands, or septations due to higher protein content and inflammatory debris. ### Imaging Characteristics Comparison | Feature | Transudative | Exudative | |---------|--------------|----------| | **Ultrasound appearance** | Anechoic, homogeneous | Echogenic, internal echoes, septations | | **CT attenuation** | Low density (0–20 HU) | Higher density (20–40 HU), may enhance | | **Loculation** | Rare, free-flowing | Common, loculated | | **Depth** | Variable, not discriminatory | Variable, not discriminatory | | **Distribution** | Often unilateral or bilateral | Often unilateral | **High-Yield:** The presence of **internal echoes, septations, or fibrin strands on ultrasound** is the single best imaging discriminator for exudative effusion. Transudates are typically simple, anechoic collections. ### Clinical Context In this patient with cirrhosis, the effusion is likely transudative (due to portal hypertension and hypoproteinemia). However, if ultrasound showed internal echoes or loculation, an exudative process (infection, malignancy, or spontaneous bacterial peritonitis) should be suspected. **Clinical Pearl:** Ultrasound is superior to plain radiography for characterizing effusion composition; it can detect as little as 50 mL of fluid and assess echogenicity in real time. [cite:Harrison 21e Ch 297]