transudative effusion

**Pathophysiology:** Starling forces are disrupted (↑ hydrostatic pressure or ↓ plasma oncotic pressure) while the capillary endothelium stays intact. Fluid filters out, but protein and cells stay behind. The result is an ultrafiltrate: clear, straw-colored, low in protein and LDH, with few cells.

**Light's criteria (pleural fluid is 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

A transudate meets none of these. Other typical transudate findings: pleural fluid protein < 3 g/dL, pH 7.4–7.55, glucose similar to serum, WBC < 1000/µL.

**Common causes:** - ↑ Hydrostatic pressure: congestive heart failure (the most common cause of transudative pleural effusion, usually bilateral or right-sided), constrictive pericarditis, volume overload. - ↓ Oncotic pressure: cirrhosis (hypoalbuminemia, plus ascites crossing the diaphragm as hepatic hydrothorax), nephrotic syndrome, protein-losing enteropathy, malnutrition. - Others: peritoneal dialysis, hypothyroidism, early atelectasis or PE (can be transudative), superior vena cava obstruction, urinothorax, and Meigs syndrome (ovarian fibroma, ascites, and pleural effusion).

**Exudate comparison:** Exudates result from ↑ capillary permeability from inflammation, infection, or malignancy. Examples are pneumonia (parapneumonic effusion), TB, malignancy, PE with infarction, pancreatitis, and autoimmune disease (RA, SLE). They have high protein and LDH and meet Light's criteria. Lymphatic obstruction can cause chylothorax (high triglycerides, > 110 mg/dL).

**Clinical significance:** - Symptoms: dyspnea, pleuritic chest pain (less common), dullness to percussion, decreased tactile fremitus and breath sounds, and egophony above the effusion. CXR shows blunting of the costophrenic angle (about 200–300 mL needed on PA film). A lateral decubitus film shows layering fluid if > 10 mm. - Management: treat the underlying cause (diuresis for CHF, albumin and diuresis for cirrhosis, etc.). Thoracentesis is mainly diagnostic when the cause is unclear, or therapeutic for symptomatic relief. Transudates generally do not need chest tube drainage. - Diuretics can falsely raise pleural fluid protein and LDH, so a transudate may be misclassified as an exudate by Light's criteria. In this setting, use serum-pleural albumin gradient > 1.2 g/dL, or pleural NT-proBNP, to support a transudate. - Ascites uses SAAG (serum-ascites albumin gradient) instead: SAAG ≥ 1.1 g/dL indicates portal hypertension (cirrhosis, CHF, Budd-Chiari), and SAAG < 1.1 g/dL indicates non-portal causes (TB peritonitis, malignancy, nephrotic syndrome, pancreatitis).

**High-yield pearl:** Bilateral effusions with cardiomegaly point to CHF (transudate). A unilateral effusion with fever and pneumonia is likely parapneumonic (exudate).