Pleural Disease: Fluid Analysis, Drainage, and Air
Thoracentesis first
Light's criteria classify an exudate when any one of the following holds: pleural fluid protein/serum protein >0.5, pleural fluid LDH/serum LDH >0.6, or pleural fluid LDH >two-thirds the upper limit of normal serum LDH. Transudates come from hydrostatic or oncotic forces (heart failure, cirrhosis, nephrotic syndrome); exudates come from infection, malignancy, pulmonary embolism, pancreatitis, and connective tissue disease. Heart failure treated with diuretics can produce exudate-like numbers - a common trap; a serum-to-pleural protein gradient >3.1 g/dL supports a transudate.
Parapneumonic effusions
Pleural fluid pH <7.20, glucose <60 mg/dL, positive Gram stain or culture, or frank pus (empyema) means the effusion is complicated and needs chest tube drainage plus antibiotics. Loculated collections may need intrapleural fibrinolytics with DNase or surgical decortication. Chylothorax shows triglycerides >110 mg/dL and is associated with lymphoma and thoracic surgery.
Pneumothorax
Tension pneumothorax is a clinical diagnosis requiring immediate needle decompression, never a chest radiograph first. Primary spontaneous pneumothorax in a stable patient with a small collection may be observed; larger or symptomatic collections are aspirated or drained. Recurrence prevention includes pleurodesis or surgery. Secondary pneumothorax in underlying lung disease is tolerated poorly and generally needs drainage and admission.
Worked rule
A patient with known heart failure on furosemide has fluid meeting exudate criteria: check the serum-pleural protein gradient rather than assuming empyema.