Light's Criteria for Exudative Effusions
Light’s criteria can be used to determine the type of a patient’s pleural effusion and thus its etiology.
The following diseases typically are exudative effusions, but in certain cases may be transudative:
- Constrictive pericarditis
- Pulmonary embolism
- Trapped lung
Light’s criteria are more sensitive than specific for exudative effusions.
Calculation of Light’s criteria provides a systematic, validated approach to evaluating pleural fluid studies. It can save the clinician significant time and avoid unnecessary additional workup. Remember, however, that Light’s criteria is more sensitive than specific test 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.