Light criteria

**Criteria (exudate if ANY one is met):** 1. Pleural fluid protein / serum protein ratio > 0.5 2. Pleural fluid LDH / serum LDH ratio > 0.6 3. Pleural fluid LDH > 2/3 the upper limit of normal for serum LDH

**Pathophysiology:** - Transudate: caused by altered Starling forces (↑ hydrostatic pressure or ↓ oncotic pressure) with an intact pleural capillary barrier. Fluid is ultrafiltrate, low in protein and LDH. - Exudate: caused by increased capillary permeability and/or impaired lymphatic drainage from local inflammation, infection, or malignancy. Fluid is protein- and LDH-rich.

**Transudate causes:** Congestive heart failure (most common), cirrhosis (hepatic hydrothorax), nephrotic syndrome (↓ albumin), hypoalbuminemia, peritoneal dialysis, myxedema, and constrictive pericarditis.

**Exudate causes:** Parapneumonic effusion/empyema, malignancy (lung, breast, mesothelioma), tuberculosis, pulmonary embolism, rheumatoid arthritis and SLE, pancreatitis, and post-MI (Dressler) syndrome.

**Additional pleural fluid findings (high-yield):** - Complicated parapneumonic effusion/empyema: pH <7.20, glucose <60 mg/dL, and LDH elevated. This needs chest tube drainage. - Low glucose (<60) with an exudate: rheumatoid pleurisy (very low), empyema, malignancy, TB. - Elevated amylase: pancreatitis or esophageal rupture. - Triglycerides >110 mg/dL: chylothorax (thoracic duct injury, lymphoma). - Bloody effusion (hematocrit >50% of blood): hemothorax (trauma). - Lymphocyte-predominant exudate: TB or malignancy. Elevated adenosine deaminase (ADA) suggests TB.

**Limitations/clinical pearls:** - Light criteria are highly sensitive for exudates (~98%) but can misclassify about 25% of transudates as exudates, especially in patients on diuretics (diuresis concentrates protein and LDH). - If a patient with presumed CHF on diuretics is misclassified as having an exudate, check the serum-effusion albumin gradient (serum albumin minus pleural albumin >1.2 g/dL suggests a transudate) or pleural fluid NT-proBNP. - Thoracentesis is indicated for new effusions, unless clearly due to CHF with a bilateral, symmetric presentation. - Imaging: effusions blunt the costophrenic angle on CXR (about 200–300 mL on upright PA film). Lateral decubitus films show fluid layering >1 cm. Physical exam findings include dullness to percussion, decreased tactile fremitus, and decreased breath sounds. - Management: treat the underlying cause for transudates. Exudates require workup (cell count, Gram stain/culture, cytology, pH, glucose).