Pleural fluid analysis (pleural effusion)

Pleural fluid analysis (1,2,3)

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

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

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

• 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

Reference:

1. Light RW. Pleural effusions. Med Clin North Am. 2011 Nov;95(6):1055-70.
2. Expert Panel on Thoracic Imaging, Morris MF, Henry TS, et al. ACR Appropriateness Criteria® Workup of pleural effusion or pleural disease. J Am Coll Radiol. 2024 Jun;21(6):S343-52.
3. Roberts ME, Rahman NM, Maskell NA, et al. British Thoracic Society guideline for pleural disease. Thorax. 2023 Jul;78(suppl 3):s1-42.