Pleural Effusion in Pneumonia: Exudative
Parapneumonic pleural effusions are always exudative, representing inflammatory fluid accumulation driven by increased pleural microvascular permeability and inflammation in response to underlying pneumonia. 1
Pathophysiological Classification
Parapneumonic effusions are inflammatory exudates dominated by polymorphonuclear leucocytes, not transudates. 1 The inflammatory process associated with pneumonia leads to accumulation of fluid with elevated protein and lactate dehydrogenase (LDH) levels, meeting Light's criteria for exudative effusions. 1
Diagnostic Confirmation
When pleural fluid is sampled from a patient with pneumonia, the following characteristics confirm the exudative nature:
- Light's criteria will be positive (pleural fluid to serum protein ratio >0.5, pleural fluid to serum LDH ratio >0.6, or pleural fluid LDH >0.67 upper limit of normal serum value). 1
- Pleural fluid LDH >67% the upper limit of normal or cholesterol >55 mg/dL confirms exudate even without serum samples. 1
- The absolute protein values help confirm exudative nature but do not predict whether the effusion will resolve spontaneously or require drainage. 1
Clinical Staging of Parapneumonic Effusions
All parapneumonic effusions exist on an exudative spectrum with three stages:
- Exudative stage: Clear fluid with low white cell count (simple parapneumonic effusion). 1
- Fibropurulent stage: Fibrin deposition with septations, increased white cells, thickening fluid (complicated parapneumonic effusion) progressing to frank pus (empyema). 1
- Organizational stage: Fibroblast infiltration creating thick, non-elastic pleural peels. 1
Critical Management Distinction
The key clinical question is not whether the effusion is exudative (it always is), but whether it is complicated and requires drainage. 1, 2 Approximately 40% of pneumonia patients develop parapneumonic effusions, but only a minority require intervention beyond antibiotics. 3, 4
Indications for immediate chest tube drainage include:
- Frankly purulent or turbid/cloudy pleural fluid on aspiration. 1, 2
- Positive Gram stain or culture from non-purulent fluid. 1
- Pleural fluid pH <7.2. 1, 2
- Pleural fluid LDH >1000 IU/L. 2
Common Pitfall
Do not confuse the question of "transudate versus exudate" with the clinically relevant question of "simple versus complicated parapneumonic effusion." 1 While all parapneumonic effusions are exudative by definition, the majority of simple parapneumonic effusions resolve with antibiotics alone and do not require drainage. 1