empyema vs pleural effusion

empyema vs pleural effusion

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:

Shape and location

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 .  
  • lenticular in shape (biconvex), whereas pleural effusions are crescentic in shape (i.e. concave towards the lung)

Findings on CT

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
  • white cells or organisms present
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