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Pleural Fluid Analysis – Maximizing Diagnostic Yield

Written by Peter Liu 

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This CHEST review summarizes common considerations and pitfalls in pleural fluid analysis.

Pleural fluid analysis – common considerations and pitfalls
Diagnosing the cause of pleural effusions usually relies on both the clinical story and pleural fluid analysis, though some etiologies can be confidently deduced with only one or the other.

Deciding on the procedure

  1. Thoracentesis can/should be deferred for several situations:
    1. The cause is evident by story alone, and empiric treatment results in clinical improvement.
    2. The effusion is too small to safely tap (<1cm in depth).
    3. Generally small ipsilateral pleural effusions associated with pulmonary embolism do not warrant thoracentesis.
  2. For challenging cases, repeat thoracentesis or pleural biopsy can be considered as a second procedure.
  3. Generally start by tapping just one side: either the larger effusion or the effusion closest to lung parenchymal abnormalities.

Transudate vs exudate

  • Light’s criteria (comparison of pleural fluid-to-serum LDH and protein) remains the most common way to distinguish between transudates and exudates. It is highly sensitive (98%) but not as specific (72%).
  • Roughly 30% of transudates are misclassified as “pseudoexudates.” These are most commonly related to diuretic use, traumatic tap, and post-CABG.
  • “Protein-discordant” (high protein, low LDH) and “LDH-discordant” (high LDH, low protein) effusions have particular differential diagnoses and are worth identifying when interpreting labs.

Pleural fluid testing and interpretation

  1. Standard tests include LDH, protein, cell count, pH, and culture. Note: pH can be challenging at some institutions due to need for timely specimen handling and testing.
  2. Additional tests to consider
    • ADA: can rule out TB in non-endemic areas with low pretest probability.  It’s fairly nonspecific and insensitive.
    • Triglycerides, chylomicrons, and cholesterol: for chylothorax and pseudochylothorax
    • Amylase: for esophageal rupture and pancreatitis-related effusions
    • Creatinine: for urinothorax
    • Glucose: broadly helpful for infection, malignancy, autoimmune, and iatrogenic etiologies
    • Hematocrit: hemothorax
    • Bilirubin: bilothorax
    • Beta-2 transferrin: CSF in pleura
    • Cytology and flow cytometry: malignancy and hematologic malignancy, respectively.
  3. Some lab findings are pathognomonic or strongly suggestive of etiology, including:
    • Glucose<5: rheumatoid effusion or empyema
    • Very high glucose: pleuro-peritoneal dialysate leak
    • LDH>1000: rheumatoid effusion
    • Protein>7: myeloma
  4. Gross fluid appearance and odor can be pathognomonic or strongly suggestive of etiology
    • Examples include hemothorax, chylothorax, empyema, esophageal rupture, urinothorax, fungal infection, and amebic liver abscess rupture
    • Color can be helpful: bloody, green, black, and milky white effusions each have a short differential diagnosis.

    How does this change my practice?
    While the majority of pleural effusions are straightforward to manage, I frequently encounter challenging decisions: 1) Should we perform thoracentesis; 2) What tests should we send; 3) What diagnoses are most likely, and which are ruled out definitively by pleural fluid testing? This review is a great resource to guide decision-making for these challenging cases.

      Source
      Pleural Fluid Analysis: Maximizing Diagnostic Yield in the Pleural Effusion Evaluation. Chest. 2025 Jun 14:S0012-3692(25)00694-4. doi: 10.1016/j.chest.2025.06.001. Epub ahead of print. PMID: 40523559

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