Light's Criteria for Exudative Effusions

A thoracentesis is typically indicated if a clinically significant pleural effusion is present that is radiographically at least 10mm thick.

• A transudative effusion occurs due to an imbalance between the hydrostatic and oncotic pressure.
• An exudative effusion, however, represents an alteration of the local factors that then precipitates a pleural fluid accumulation.

• Perform a diagnostic and therapeutic needle thoracentesis or chest tube drainage of pleural effusion.
• Obtain pleural fluid and serum studies of protein and LDH.
• Consider additional pleural fluid studies (cell count, differential, culture, cytology, triglycerides).
• Determine if pleural fluid is exudative by meeting at least one of Light’s criteria:
  • Pleural fluid protein / Serum protein >0.5
  • Pleural fluid LDH / Serum LDH >0.6
  • Pleural fluid LDH > 2/3 * Serum LDH Upper Limit of Normal
• Review table 1 below to narrow differential:

• Fluid color itself can also assist in suggesting a potential etiology as in table 2:

Proper diagnosis of the underlying etiology is important as the treatments for the numerous exudative and transudative etiologies differ significantly. Typically, exudative effusions require a further investigative workup which may include cytopathology studies, biopsy, or even a thoracotomy. Conversely, transudative effusions usually resolve with treatment of the underlying condition.