Pleural Effusions

Table of Contents

Table of Contents

Updated April 2022

Authors: Neha Deshpande, MD1,  Ryan Murphy, MD2
Executive Editor: Yilin Zhang, MD1

1 Assistant Professor, Department of Medicine, University of Washington
2 Clinical Instructor, Department of Medicine, Division of Pulmonary and Critical Care Medicine


  1. Describe criteria differentiating pleural effusions into transudative and exudative effusions. 
  2. Generate a differential diagnosis for each transudative and exudative effusions and associate diagnoses with characteristic pleural fluid studies. 
  3. Create a framework for the management of parapneumonic effusions. 

Teaching Instructions

Plan to spend at least 30-60 minutes preparing for this talk by using the Interactive Board and the Teaching Instructions. Click through the graphics animations to become familiar with the flow and content of the talk. All clickable elements will be denoted by a shaded rounded rectangle and a mouse cursor. 

Begin with reviewing the objectives for the session. We recommend progressing in order, though this gives you the flexibility to deliver more focused teaching. The talk can be presented in one of two ways:

  1. Project the “Interactive Board” OR
  2. Reproduce your own drawing of the presentation on a whiteboard/ Chalkboard

The anticipated time for the content of this talk is about 15 min. With cases, 25-30 min. 

Title PageIntroduce the ways to identify a pleural effusion on imaging (blunted costophrenic angle, meniscus on CXR). Ask your learners which pleural effusions warrant further evaluation. Click on the button to reveal the answer

Objective 1 – Describe criteria differentiating pleural effusions into transudative and exudative effusions. (Light's Criteria)

The first step in the evaluation of a pleural effusion is to perform an ultrasound-guided diagnostic thoracentesis and obtain pleural fluid. Light’s criteria compare the pleural fluid LDH and protein to serum LDH and protein and differentiate transudative (non-inflammatory) from exudative (inflammatory) effusions. Light's criteria has a sensitivity of 98% and specificity of 83%. However, it is not accurate in the setting of diuretic use or in with high pleural RBC counts (> 10k) which can falsely elevate LDH. 

Bonus learning: The serum albumin – pleural albumin gradient can also be used to identify a transudative process (gradient > 1.2). 

Additional standard and specialized pleural fluid studies can further characterize the effusion and help diagnose the underlying etiology. Ask your learners to review Light’s criteria (click “Light's criteria” to reveal). Ask them how many of Light’s criteria need to be met in order to define an exudative process. Then, ask your learners which additional pleural fluid studies they would send. Click on “pleural studies” and “specialized tests” to reveal additional information.  

Bonus learning: To learn more about any of these studies, you may click on the “bonus information” button to navigate to a page with detailed information on each test.

Objective 2 – Challenge your learners to list off some causes of transudative and exudative effusions before moving onto the differential slide. (Differential)

Navigate to the Differential slide by either clicking on the “Differential” page in the left hand table of contents or on the “transudate” or “exudate” buttons. Challenge your learners to list off some causes of transudative and exudative effusions before moving onto the differential slide

  • Transudative effusionsClick on “transudate” to reveal a differential. Typically caused by volume overload (heart failure, renal failure) or cirrhosis. Hypoalbuminemia can also result in third spacing to the pleural space.  
  • Exudative effusionsClick on “exudate” to reveal a differential. Exudative effusions in particular are associated with a variety of diseases and warrant further diagnostic evaluation to determine the specific cause. Of note, this is not an exhaustive list, and this talk does not discuss exceptional cases when transudative effusions can be mischaracterized as exudative (i.e. heart failure and cirrhosis treated with diuretics). Differentiate between “infection” in which the pleural space itself is infected, and “parapneumonic,” which is a reactive, pleural inflammatory process caused by a pulmonary infection or pneumonia. Click on each cause of exudative effusion to review characteristic cell counts (neutrophilic or lymphocytic predominance) and other diagnostic pleural fluid studies. Special teaching points are included below:
    • Mycobacterial tuberculosis pleural effusions, AFB stain and culture from pleural fluid are not sensitive, and pleural biopsy for tissue stain and culture is diagnostic. An adenosine deaminase study can also be suggestive of a tuberculous infection. 
    • For malignant effusions, higher volume therapeutic thoracenteses increase the diagnostic yield. The sensitivity of an initial thoracentesis for malignant effusion is ~ 60%. The sensitivity increases to ~ 75% with a second large volume thoracentesis.

Objective 3 – Create a framework for the management of parapneumonic effusions. (Management)

Further characterization of parapneumonic effusions into uncomplicated and complicated effusions is critical because this distinction determines management. Ask your learners how to classify parapneumonic effusions based on the presence of septations and/or loculations on imaging, low pleural fluid pH and glucose, and frank pus and/or bacteria on gram stain (click on each of these pleural findings to reveal classification of parapneumonic effusions)

  • Uncomplicated parapneumonic effusions – these are effusions without loculations, septations and do not meet other criteria for complicated parapneumonic effusion or empyema. Click on “uncomplicated parapneumonic effusion to reveal treatment. Emphasize that uncomplicated effusions are treated with antibiotics alone.
  • Complicated parapneumonic effusions – Complicated effusions have loculations, septations, low pH < 7.2 or low glucose. Click on complicated effusion to reveal management.
  • Empyema – Empyema is diagnosed based on presence of frank pus or positive pleural culture. Contrast the management of complicated effusions and empyema from uncomplicated effusions. Complicated effusions and empyemas require drainage of the pleural space for effective treatment. The treatment of a complicated pleural effusion is multi-disciplinary, and often needs interventional radiology, pulmonology, and thoracic surgery consultation.

Ask learners to list mechanisms of complex pleural fluid drainage. Click on “antibiotics and drainage” to reveal additional information. Depending on the complexity of the effusion, less-invasive pigtail placement may be sufficient. For heavily loculated effusions, a larger-bore chest tube may be more appropriate. For empyema, more-invasive surgical procedures such as video-associated thoracic surgery (VATS) or open decortication are likely to be needed. 

Instillations of tPA and dornase through the pleural catheter can augment drainage, however, this has not been shown to decrease mortality, and more data about the optimal timing of surgery is needed. A randomized controlled trial comparing outcomes of fibrinolytic therapy to early VATS decortication is planned. 

Cases – There are a total of 3 cases. After reading through the stem, have your learners request work-up and click on the corresponding section. Not all tests are needed in every case.

Interactive Board

Take Home Points

  1. Light’s criteria are used to distinguish transudative and exudative pleural effusions. 
  2. Transudative effusions develop from non-inflammatory conditions (such as heart, renal, and liver failure) and exudative effusions typically result from inflammatory diseases (such as infections, malignancy, autoimmune diseases).
  3. Parapneumonic effusions are further differentiated into complicated and uncomplicated effusions by the presence of loculations, low pH, and low glucose. Complicated parapneumonic effusions and empyema require drainage for effective treatment and consultation with multiple subspecialties. 


  1. Light RW. Clinical practice. Pleural effusion. NEJM. 2002; 346(25):1971-1977. 
  2. Feller-Kopman D and Light R. Pleural disease. NEJM. 2018;378(8):740-51. DOI: 10.1056/NEJMra1403503. 
  3. Porcel JM and Light RW. Pleural effusions. Disease-a-Month. 2013;59:29-57. DOI: 10.1016/j.disamonth.2012.11.002. 
  4. [Internet]. Bethesda (MD): National Library of Medicine (US). 2018 Jul 26 – . Identifier NCT03583931, Early Intervention for complicated parapneumonic effusion: randomized controlled trial for fibrinolytic therapy versus VATs decortication; [cited 2020 Dec 16]; [about 8 pages]. Available from: 
  5. Heffner JE. Diagnostic evaluation of a pleural effusion in adults: Initial testing. In Uptodate, Finaly G (Ed), UpToDate, Waltham, MA. (Accessed on Dec 17, 2020).
Yilin Zhang