Pleural fluid (summary)

Dr Jeremy Jones et al.

Pleural fluid describes fluid within the pleural space. Fluid may be simple fluid, pus, haemorrhage 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 oedema
      • pus
      • haemorrhage
  • investigation
    • chest x-ray
      • first line test for most chest pathology
      • identifies moderate and large effusions
    • CT chest
      • is much more sensitive
      • not usually used for primary assessment of an effusion
      • CT may be used where there is pleural infection
        • helps to confirm empyema
        • helps to identify loculations
        • helps to identify regions that may be drained
Plain radiograph

The costophrenic angle is an important area to review on the chest x-ray. It should be crisp - when it is not, there is either a technical or pathological reason for the loss of the normal crisp angle.

The commonest cause is fluid collecting in the pleural space which results in blunting of the angle. However, about 300-500 mL of fluid is required in the pleural space before it can be visible. Hence, why patients are often symptomatic.

Other findings on the chest x-ray may help you to narrow down the differential diagnosis, e.g. pulmonary oedema in heart failure, airspace opacification in infection, or a mediastinal mass in cancer.


The volume of fluid that can be detected at CT is much less than on a chest x-ray.

CT will confirm fluid within the pleural space, but can also help to determine the cause. The density of fluid can help determine if it is simple fluid or blood. If there is thickening of the pleura and enhancement in the context of infection, empyema is more likely. Septations within the fluid make infection more likely.

Medical student radiology curriculum
Share article

Article information

rID: 51425
System: Chest
Synonyms or Alternate Spellings:

Support Radiopaedia and see fewer ads

Cases and figures

  • Drag
    Case 1: right pleural effusion
    Drag here to reorder.
  • Drag
    Case 2: simple pleural effusion
    Drag here to reorder.
  • Drag
    Case 4: empyema with enhancing margins
    Drag here to reorder.
  • Drag
    Case 3: haemothorax with increased density
    Drag here to reorder.
  • Updating… Please wait.

    Alert accept

    Error Unable to process the form. Check for errors and try again.

    Alert accept Thank you for updating your details.