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Pleural fluid analysis (pleural effusion)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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pleural fluid analysis

Gross appearance of the pleural fluid should be recorded in order to identify potential aetiologies:

  • putrid odour - anaerobic empyema
  • food particles – oesophageal
  • anchovy brown fluid - ruptured amoebic abscess
  • bile staining - cholothorax (biliary fistula)
  • milky - chylothorax/pseudochylothorax
  • black fluid – Aspergillus infection (1,2)

Pleural fluid tests include:

  • recommended for all samples
  • biochemistry - LDH and protein, blood should be sent simultaneously to biochemistry for total protein and LDH so that Light's criteria can be applied
  • microbiology - for microscopy, culture and sensitivities, in case of suspected pleural infection, additional samples of blood culture bottles should be sent
  • cytological examination and differential cell count - refrigerate if delay in processing anticipated (eg, out of hours)
  • additional tests for selected cases
  • pH - in non-purulent effusions when pleural infection is suspected
  • glucose - low in effusions due to rheumatoid arthritis, tuberculosis, SLE and malignancy
  • gram and auramine (or Ziehl-Neelson) stain
  • triglycerides and cholesterol - to differentiate chylothorax from pseudochylothorax in milky effusions
  • amylase - occasionally useful in suspected pancreatitis-related effusion.
  • haematocrit- diagnosis of haemothorax (1,3)

Light's criteria is used to differentiate between an exudate and transudate pleural effusion (1)

  • in order to apply Light's criteria, the total protein and LDH should be measured in both blood and pleural fluid
  • pleural fluid is an exudate if one or more of the following are met
    • pleural fluid protein divided by serum protein is >0.5
    • pleural fluid lactate dehydrogenase to serum lactate dehydrogenase ratio >0.6
    • pleural fluid level more than two thirds of the normal upper value for serum lactate dehydrogenase as determined locally (1,3)

Reference:


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