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Light criteria in pleural effusion

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

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)


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