Light's Criteria for Exudative Effusions
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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:
|Meigs syndrome||CSF leak into pleural space|
|Eosinophilic granulomatosis with polyangiitis||Hypoalbuminemia|
|Granulomatosis with polyangiitis||Nephrotic syndrome|
- Fluid color itself can also assist in suggesting a potential etiology as in table 2:
|Fluid Appearance/Odor||Necessary Fluid Study||Differential|
|Bloody||Hematocrit and RBC count||Malignancy, trauma, PE, hemothorax|
|Putrid odor||Gram stain and culture||Anaerobic infection|
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.