Thoracentesis Laboratory Medicine

Updated: Sep 11, 2020
  • Author: Mark E Brauner, DO; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
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Laboratory Medicine

Laboratory Medicine Summary

Diagnostic analysis of pleural fluid

Pleural fluid is labeled and sent for diagnostic analysis. If the effusion is small and contains a large amount of blood, the fluid should be placed in a blood tube with anticoagulant so that it does not clot. The following laboratory tests should be requested:

  • pH level

  • Gram stain, culture

  • Blood cell count and differential

  • Glucose level, protein levels, and lactic acid dehydrogenase (LDH) level

  • Cytology

  • Creatinine level if urinothorax is suspected (eg, after an abdominal or pelvic procedure)

  • Amylase level if esophageal perforation or pancreatitis is suspected

  • Triglyceride levels if chylothorax is suspected (eg, after coronary artery bypass graft [CABG], especially if the inferior mesenteric artery [IMA] was used; milky appearance is not sensitive)

Exudative pleural fluid can be distinguished from transudative pleural fluid by looking for the following characteristics (exudates have 1 or more of these characteristics, whereas transudates have none):

  • Fluid/serum LDH ratio ≥0.6

  • Fluid/serum protein ratio ≥0.5

  • Fluid LDH level within the upper two thirds of the normal serum LDH level