Empyema vs pleural effusion

Last revised by Assoc Prof Craig Hacking on 17 Apr 2020

An empyema can resemble a pleural effusion and can mimic a peripheral pulmonary abscess. Features that help distinguish a pleural effusion from an empyema include:

Empyemas usually:

  • form an obtuse angle with the chest wall
  • unilateral or markedly asymmetric whereas pleural effusions are (if of any significant size) usually bilateral and similar in size 4.  
  • lenticular in shape (biconvex), whereas pleural effusions are crescentic in shape (i.e. concave towards the lung)

Features suggestive of an empyema include:

  • enhancing thickened pleura (see split pleura sign) whereas pleural effusion has thin imperceptible pleural surfaces
  • locules of gas absent unless recent thoracocentesis
  • obvious septations
  • associated consolidation
  • associated adjacent infection (e.g. subdiaphragmatic abscess)
  • pH <7.0
  • glucose level <40mg/dL 4
  • white cells or organisms present

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Cases and figures

  • Pleural empyema
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