exudative effusion

**Pathophysiology:** Exudates form when local inflammation, infection, or tumor increases pleural capillary permeability, or when lymphatic drainage is obstructed. Protein, LDH, cells, and inflammatory mediators leak into the pleural space. Transudates instead form from Starling force imbalance (↑ hydrostatic pressure or ↓ oncotic pressure) with intact capillaries.

**Light's criteria (exudate if ANY one is met):** - Pleural fluid protein / serum protein > 0.5 - Pleural fluid LDH / serum LDH > 0.6 - Pleural fluid LDH > 2/3 the upper limit of normal serum LDH

Other findings that favor exudate: pleural fluid cholesterol > 45–55 mg/dL, and serum–pleural albumin gradient ≤ 1.2 g/dL. Light's criteria are very sensitive for exudates but can misclassify transudates as exudates, especially in patients on diuretics. In that setting, use the serum–pleural albumin gradient or protein gradient.

**Common causes:** - **Infection:** Parapneumonic effusion (uncomplicated, complicated, or empyema) and TB pleuritis. - **Malignancy:** Lung, breast, lymphoma, and mesothelioma. Fluid may be bloody, and cytology may be positive. - **Pulmonary embolism** - **Autoimmune disease:** RA (very low glucose, low pH) and SLE (drug-induced lupus too). - **Pancreatitis:** Amylase-rich, usually left-sided. - **Others:** Post-MI (Dressler syndrome), asbestos exposure, uremia, drugs (amiodarone, nitrofurantoin, methotrexate), and chylothorax (triglycerides > 110 mg/dL, from thoracic duct injury or lymphoma).

**Special pleural fluid findings:** - **Complicated parapneumonic effusion or empyema:** pH < 7.2, glucose < 60 mg/dL, positive Gram stain or culture, or frank pus. These need chest tube drainage. - **TB:** Lymphocyte-predominant, ↑ adenosine deaminase (ADA). - **RA:** Glucose < 30 mg/dL. - **Malignancy:** Bloody, with positive cytology. - **Pancreatitis or esophageal rupture:** ↑ amylase. - **Eosinophilic effusion (> 10% eosinophils):** Air or blood in the pleural space, drugs, asbestos, or parasites.

**Clinical presentation:** Dyspnea, pleuritic chest pain, and cough. Exam shows decreased breath sounds, dullness to percussion, decreased tactile fremitus, and egophony above the effusion. Imaging: blunting of the costophrenic angle on upright CXR (needs ~200–300 mL), and layering on lateral decubitus film (> 10 mm). Ultrasound or CT is more sensitive.

**Management:** Diagnostic thoracentesis, then treat the underlying cause. Complicated effusions or empyema need drainage, with fibrinolytics or surgery (VATS/decortication) if loculated. Malignant effusions that recur may need indwelling pleural catheter or pleurodesis.

**High-yield pearl:** Transudate causes are systemic (CHF most common, cirrhosis/hepatic hydrothorax, nephrotic syndrome), while exudate causes are local (disease of the pleura or adjacent lung). If the question stem mentions infection, cancer, PE, or autoimmune disease, think exudate.