pleural fluid analysis (pleural effusion)

Last reviewed 01/2021

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:

• (1) Hooper C et al. Investigation of a unilateral pleural effusion in adults: British Thoracic Society Pleural Disease Guideline 2010. Thorax. 2010;65 Suppl 2:ii4-17
• (2) Saguil A, Wyrick K, Hallgren J. Diagnostic approach to pleural effusion. Am Fam Physician. 2014;90(2):99-104
• (3) Bhatnagar R, Maskell N. The modern diagnosis and management of pleural effusions. BMJ. 2015;351:h4520.