What are the types of pleural effusion?

Types of Pleural Effusion

Pleural effusions are primarily classified into two main types: transudates and exudates, based on their formation mechanism and biochemical characteristics. 1, 2

Transudative Pleural Effusions

Transudative effusions occur when systemic factors alter the balance of hydrostatic forces influencing pleural fluid formation and absorption, favoring fluid accumulation while the permeability of capillaries to proteins remains normal 1, 3.

Common causes of transudative pleural effusions include:

• Heart failure (accounts for >80% of transudates) 2, 3
• Cirrhosis with ascites (approximately 10% of transudates) 2, 3
• Renal failure 1
• Hypoalbuminemia 1
• Peritoneal dialysis 3
• Nephrotic syndrome 3

Exudative Pleural Effusions

Exudative effusions develop when the pleural surface and/or local capillary permeability are altered, allowing protein-rich fluid to accumulate 1, 4.

Common causes of exudative pleural effusions include:

• Pneumonia (parapneumonic effusions) 1, 4
• Malignancy (lung cancer, breast cancer, lymphomas) 4, 5
• Tuberculosis 6
• Pulmonary embolism 1, 4
• Autoimmune disorders 1

Diagnostic Criteria for Classification

The European Respiratory Society recommends using Light's criteria as the gold standard for differentiating exudates from transudates 6, 2:

An effusion is classified as exudative if it meets ANY ONE of the following:

• Pleural fluid/serum protein ratio > 0.5 6, 2
• Pleural fluid/serum LDH ratio > 0.6 6, 2
• Pleural fluid LDH > 67% of the upper limit of normal for serum LDH 6, 2

Light's criteria have a sensitivity of 98% and specificity of 72% for identifying exudative effusions 2.

Special Types of Pleural Effusions

Based on appearance and characteristics, pleural effusions may also be classified as:

• Empyema: Purulent pleural fluid containing gross pus, positive Gram stain, glucose <40 mg/dL, or pH <7.00 4
• Chylothorax: Milky appearance due to high lipid content 1
• Hemothorax: Bloody effusion with a pleural fluid hematocrit that should be measured for diagnosis 1
• Pseudochylothorax: Turbid, milky appearance that remains after centrifugation due to high lipid content 1

Common Pitfalls in Classification

• Relying solely on visual or radiological assessment is unreliable for transudate-exudate differentiation 2
• Some transudates may be misclassified as exudates by Light's criteria, particularly in patients on diuretics 2, 7
• For misclassified effusions, the albumin gradient (serum albumin minus pleural fluid albumin > 1.2 g/dL) can correctly reclassify approximately 80% of "false" exudates 2
• NT-proBNP measurement in pleural fluid or serum can help identify heart failure-related effusions that may be misclassified 2

Diagnostic Approach

1. Clinical assessment to determine if the effusion is likely transudative or exudative 1
2. Pleural fluid aspiration for biochemical analysis using Light's criteria 1, 6
3. Additional tests based on suspected etiology:
   • Cytology for suspected malignancy 6, 5
   • Gram stain and culture for suspected infection 1
   • AAFB stain and culture for suspected tuberculosis 1
   • pH measurement for suspected infection 1, 6

Proper classification of pleural effusions guides further diagnostic workup and management, with exudative effusions typically requiring more extensive investigation for underlying causes 2.