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    Light's Criteria for Exudative Effusions

    Provides Light's Criteria to help determine if pleural fluid is exudative.
    When to Use
    Why Use

    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:

    • Amyloidosis
    • Chylothorax
    • Constrictive pericarditis
    • Malignancy
    • Pulmonary embolism
    • Sarcoidosis
    • 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.

    Protein Parameters
    LDH Parameters


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    Next Steps
    Creator Insights


    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:
    Exudative Transudative
    Malignancy Heart failure
    ARDS Atelectasis
    Meigs syndrome CSF leak into pleural space
    Pancreatitis Hepatic hydrothorax
    Eosinophilic granulomatosis with polyangiitis Hypoalbuminemia
    Granulomatosis with polyangiitis Nephrotic syndrome
    Lupus Peritoneal dialysis
    Lung abscess Urinothorax

    • 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
    Cloudy Triglycerides Chylothorax
    Putrid odor Gram stain and culture Anaerobic infection

    Critical Actions

    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.

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    About the Creator
    Dr. Richard W. Light
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