Pleural fluid (summary)

Last revised by Daniel J Bell on 16 Jun 2021
This is a basic article for medical students and other non-radiologists

Pleural fluid describes fluid within the pleural space. Fluid may be simple fluid, pus, hemorrhage and therefore is a broad description that includes, but not synonymous with pleural effusion. Pus in the pleural space may become loculated and in some cases, may have solid elements.

Reference article

This is a summary article; we do not have a more in-depth reference article.

  • pathophysiology
    • fluid within the pleural space
    • causes
      • simple fluid: pleural effusion
        • high protein: exudate, e.g. infection
        • low protein: transudate, e.g. pulmonary edema
      • pus
      • hemorrhage
  • role of imaging
    • is there a pleural effusion?
      • a chest x-ray is first line test
      • US and CT are more sensitive and specific but not always required
    • are there other features that suggest a cause?
      • air-space opacification in pneumonia or heart failure
      • cardiomegaly in heart failure
      • focal lung lesion or collapse in lung cancer
    • is it complex (septations or loculations)?
      • ultrasound is best, but CT may help
    • is it infected (enhancement)? 
      • CT is best; ultrasound is less sensitive
    • what is the best way to drain it?
      • ultrasound is helpful - most drainage will be ultrasound-guided
  • common pathology
  • unilateral opacification without volume loss or air bronchograms
    • usually basal
  • blunting of the costophrenic angle 
    • this may be the only sign in smaller effusions
    • if it can be seen on a
      • frontal chest x-ray, it measures at least 300-500 mL
      • lateral chest x-ray, about 200 mL
  • fluid is dark (hypoechoic) on ultrasound
    • the underlying lung may be consolidated
  • additional features:
    • septations: parapneumonic effusion or early empyema
    • loculation: established pleural infection
    • thick turbid fluid with echogenic material: empyema
  • the volume required for detection is much smaller than CXR
  • fluid on CT is an intermediate density: 0-30 HU
    • the density may help to determine the type of fluid
      • simple fluid will have a lower HU than blood
  • if IV contrast is given, evidence of pleural enhancement may point to an infective cause

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Cases and figures

  • Case 1: right pleural effusion
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  • Case 2: simple pleural effusion
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  • Case 3: hemothorax with increased density
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