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Pleural effusion - classification

Pleural effusions are traditionally classified as either exudates or transudates but they can also contain blood (haemothorax) or chyle (chylothorax). A chylothorax usually occurs because of disruption of the thoracic duct. A pseudo chylothorax occurs secondary to a long-standing pleural effusion and is characteristered by the accumulation of cholesterol crystals.

An exudative effusion occurs when local factors are altered, such as inflammation of the lung or the pleura leading to capillary leakage of fluid into the pleural space.

A transudative effusion, by contrast, occurs when systemic factors come into play. This includes an elevated portal pressure from cirrhosis, elevated visceral pulmonary capillary pressure from left-sided heart failure, elevated parietal pleural capillary pressure from right-sided heart failure, or low oncotic pressure due to hypoalbuminaemia.

Exudates subsequently have a high protein content (>30g/L) and transudates a low protein content (

Light’s criteria state that the pleural fluid is an exudate if one or more of the following criteria are met (sensitivity 98%, specificity 83% for exudate):

  • Pleural fluid protein : serum protein > 0.5

  • Pleural fluid LDH : serum LDH > 0.6

  • Pleural fluid LDH > 2/3 upper limit of normal serum LDH

Additional criteria used to confirm exudate if results equivocal:

  • Serum albumin – pleural fluid albumin

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