Thoracentesis pleural fluid analysis interpretation

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

Transudate versus exudate
First the fluid is either transudate or 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.

Exudate 
 * hemorrhage
 * Infection
 * Inflammation
 * Malignancy
 * Iatrogenic
 * Connective tissue disease
 * Endocrine disorders
 * Lymphatic disorders vs Constrictive pericarditis

Transudate
 * Congestive heart failure
 * Nephrotic syndrome
 * Hypoalbuminemia
 * Cirrhosis
 * Atelectasis
 * trapped lung
 * Peritoneal dialysis
 * Superior vena cava obstruction

Amylase
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, cancer or esophageal rupture.

Glucose
This is considered low if pleural fluid value is less than 50% of normal serum value. The differential diagnosis for this is:
 * rheumatoid effusion
 * lupus effusion
 * bacterial empyema
 * malignancy
 * tuberculosis
 * esophageal rupture (Boerhaave syndrome)

pH
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.

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.

The main cause for chylothorax is rupture of the thoracic duct, most frequently 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 as to the type on 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
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.