Clinical meaning
Pleural effusions result from imbalance between fluid formation and absorption in the pleural space. Transudative effusions form from increased hydrostatic pressure (CHF — most common cause, hepatic hydrothorax, nephrotic syndrome) or decreased oncotic pressure (hypoalbuminemia). The pleural membrane is intact — fluid is an ultrafiltrate of plasma. Exudative effusions result from increased capillary permeability due to inflammation (pneumonia, TB), malignancy (pleural metastases, mesothelioma), or lymphatic obstruction. Light's criteria differentiate transudative from exudative: an effusion is exudative if ANY ONE criterion is met — pleural protein/serum protein > 0.5, pleural LDH/serum LDH > 0.6, or pleural LDH > 2/3 upper limit of normal serum LDH. Light's criteria have 98% sensitivity for exudates but may misclassify transudates in patients on diuretics — apply serum-to-pleural albumin gradient (> 1.2 g/dL suggests transudate despite meeting Light's criteria).
Diagnosis & workup
Diagnostics & workup: - CXR: meniscus sign (blunting of costophrenic angle at ~300 mL), lateral decubitus film to confirm free-flowing (> 10 mm layering = safe to tap) - Thoracentesis for all new unilateral effusions (except clearly bilateral in known CHF responsive to diuretics) - Light's criteria: protein ratio > 0.5, LDH ratio > 0.6, or pleural LDH > 2/3 ULN serum LDH → EXUDATE - Pleural fluid analysis: cell count and differential, protein, LDH, glucose, pH, Gram stain, culture, cytology - Low glucose (< 60 mg/dL): empyema, rheumatoid pleurisy, TB, malignancy, lupus pleuritis, esophageal rupture - Low pH (< 7.2): complicated parapneumonic effusion/empyema requiring drainage - Cytology: sensitivity 60% for first sample, 70-75% with repeat; mesothelioma requires biopsy - CT chest with contrast: parenchymal lesions, pleural thickening/nodularity (malignancy), mediastinal lymphadenopathy