Noon Report: Pleural Effusion

The Orange Team reviewed the case of a patient who presented with recurrent fevers and pneumonia, found to have an empyema on CXR and thoracentesis fluid analysis. Let’s learn a few things about pleural effusions.

The most common causes of pleural effusion are CHF, pneumonia, and malignancy. Effusions are characterized as transudative or exudative. Transudative effusions are due to an imbalance in hydrostatic and oncotic pressure (CHF, cirrhosis, pulmonary embolism). Exudative effusions due to local factors influencing fluid accumulation in the pleural space. The distinction between transudative and exudative can be made with Light’s criteria:

Light’s Criteria: Pleural effusion is exudative if one or more of the following is present

  • Pleural fluid LHD : Serum LHD > 0.6
  • Pleural fluid protein : Serum protein > 0.5
  • Pleural fluid LHD > 2/3 of upper limit of normal serum LDH level

 Images courtesy of wikicommons [2]

Images courtesy of wikicommons [2]

Interestingly, there is a caveat to Light’s criteria. If the clinical story is consistent with transudative but Light’s criteria is exudative then you should  look at the difference between the albumin levels. If the serum and pleural difference is > 1.2 g/dL then the effusion is likely transudative. This fact is particularly important since diuretics have been shown to effect Light’s criteria and thus the albumin gradient should be considered if a patient is receiving diuretics.

Here are some interesting factoids about exudative pleural effusions:

  • Monocyte predominance indicates a chronic process. Lymphocytic predominance is often seen with effusions due to malignancy and tuberculosis. Eosinophilic effusions are often due to vasculitis, drug reactions, or asbestos exposure
  • Cultures have the highest yield if collected in blood culture bottles
  • Pleural LDH level correlates with the level of pleural inflammation and can be trended if multiple thoracentesis are being performed
  • Pleural fluid with a pH < 7.2 requires drainage. This low pH if associated with a malignant effusion is a very poor prognostic sign

There is so much more to learn on pleural effusions. Two great articles can be found here and here.


  1. Light, Richard. "Pleural Effusion." NEJM. 2002; 346: 1971-77.
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