## Diagnosis of Pleural Effusion Type **Key Point:** The Light's criteria are the gold standard for distinguishing transudative from exudative effusions and must be applied using simultaneous serum and pleural fluid measurements. ### Light's Criteria An effusion is exudative if ANY of the following are true: 1. Pleural fluid LDH > 2/3 of upper limit of normal serum LDH 2. Pleural fluid protein > 3 g/dL 3. Pleural fluid LDH : serum LDH ratio > 0.6 4. Pleural fluid protein : serum protein ratio > 0.5 If NONE of these criteria are met, the effusion is transudative. ### Clinical Context This patient has cirrhosis with ascites — a classic cause of transudative pleural effusion. The effusion occurs due to: - Increased hydrostatic pressure from portal hypertension - Decreased plasma oncotic pressure from hypoalbuminemia - Direct transdiaphragmatic lymphatic flow from ascites into pleural space **High-Yield:** In cirrhosis, pleural effusion is transudative in >90% of cases. Confirming transudative nature excludes need for further invasive investigations and guides conservative management (diuretics, albumin infusion, consideration of TIPS). **Clinical Pearl:** Simultaneous serum samples are mandatory because serum LDH and protein values change with time and hydration status. Obtaining pleural fluid without paired serum values makes Light's criteria uninterpretable. ### Why This Investigation First? Light's criteria immediately classify the effusion and determine the diagnostic pathway: - **Transudative** → treat underlying cause (diuretics, albumin, TIPS) - **Exudative** → pursue cell count, culture, cytology, ADA, glucose, pH [cite:Harrison 21e Ch 297]
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