Thoracentesis pleural fluid analysis interpretation

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]

Interpretation of pleural fluid analysis

Several diagnostic tools are available to determine the etiology of pleural fluid.

Normally the pleural cavity contains less than 20 milliliters (4 teaspoons) of clear, yellowish (serous) fluid

Pleural fluid can be differentiated as

Transudate versus exudate

Light's criteria differentiates between transudate and exudate.

A transudate is defined as

  • pleural fluid to serum total protein ratio of less than 0.5,
  • pleural fluid to serum LDH ratio < 0.6 and
  • absolute pleural fluid LDH < 200 IU or < 2/3 of the normal serum level.

An exudate is any pleural fluid that does not meet aforementioned criteria.

Causes of Exudative pleural effusion:
Causes of Transudative pleural effusion:


A high amylase level (twice the serum level or the absolute value is greater than 160 Somogy units) in the pleural fluid is indicative of either acute or chronic pancreatitis, pancreatic pseudocyst that has dissected or ruptured into the pleural space, malignancy or esophageal rupture.


Pleural fluid glucose value is considered as low, if it is less than 50% of normal serum value. The differential diagnosis for low pleural fluid glucose is:


Normal pleural fluid pH is approximately 7.60. A pleural fluid pH below 7.30 with normal arterial blood pH has the same differential diagnosis as low pleural fluid glucose, i.e., commonly parapneumonic effusion and empyema.

Triglyceride and cholesterol

Chylothorax (fluid from lymph vessels leaking into the pleural cavity) may be identified by determining triglyceride and cholesterol levels, which are relatively high in lymph. A triglyceride level over 110 mg/dl and the presence of chylomicrons indicate a chylous effusion. The appearance is generally milky but can be serous.

Patients with pseudochylothorax have increased cholesterol and decreased triglyceride levels.

The main cause for chylothorax is rupture of the thoracic duct, usually as a result of trauma or malignancy (such as lymphoma).

Cell count and differential

The number of white blood cells can give an indication of infection. The specific subtypes can also give clues to the type of infection. The amount of red blood cells are an obvious sign of bleeding.

Cultures and stains

If the effusion is caused by infection, microbiological culture may yield the infectious organism responsible for the infection, sometimes before other cultures (e.g. blood cultures and sputum cultures) become positive. A Gram stain may give a rough indication of the causative organism.

A Ziehl-Neelsen stain may identify tuberculosis or other mycobacterial diseases.


Cytology is an important tool in identifying effusions due to malignancy. The most common causes for pleural fluid are lung cancer, metastasis from elsewhere and mesothelioma. The latter often presents with an effusion. Normal cytology results do not reliably rule out malignancy, but make the diagnosis more unlikely.

Adenosine deaminase level-

Measurement of adenosine deaminase may also be useful in determining the presence of tuberculosis (sensitivity and specificity > 90%, although it may also be elevated in patients with empyema or malignancy)

Hematocrit level

Hematocrit > 1% indicates possible pneumonia, pulmonary embolism, malignancy, or trauma; pleural fluid hematocrit > 0.5 × peripheral blood hematocrit indicates hemothorax

N-terminal pro-brain natriuretic peptide level

Elevated in patients with heart failure; useful in diagnosing heart failure when effusion is classified as exudative by Light's criteria

Tumor markers

May be ordered based on clinical suspicion; includes carcinoembryonic antigen, cancer antigen 125, cancer antigen 15-3, cytokeratin 19 fragment, and mesothelin testing.


  1. Mercer, Rachel M; Corcoran, John P; Porcel, Jose M; Rahman, Najib M; Psallidas, Ioannis (2019). "Interpreting pleural fluid results". Clinical Medicine. 19 (3): 213–217. doi:10.7861/clinmedicine.19-3-213. ISSN 1470-2118.

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