Workup for Moderate Pleural Effusion
The appropriate workup for a moderate pleural effusion should begin with diagnostic thoracentesis and CT chest with IV contrast to determine the underlying cause, as these procedures provide the most critical information for reducing morbidity and mortality. 1, 2
Initial Imaging
First-line Imaging
- Chest radiography (PA and lateral views): Initial imaging to confirm presence and estimate size of effusion
- Can detect effusions >175 mL on frontal view and >75 mL on lateral view 1
- Limited in determining etiology or complexity
Second-line Imaging
CT chest with IV contrast: Recommended for all moderate pleural effusions
- Optimal timing: 60 seconds after contrast bolus for best pleural visualization 1
- Helps identify:
- Pleural enhancement (sensitivity 84%, specificity 83%)
- Pleural thickening (sensitivity 68%, specificity 87%)
- Loculations (sensitivity 52%, specificity 89%)
- Extrapleural fat changes 1
- Aids in detecting underlying lung pathology (masses, pneumonia, etc.)
Ultrasound of chest:
- Superior sensitivity for detecting small effusions (can detect as little as 20 mL) 2
- Essential for guiding thoracentesis
- Helps characterize effusion:
- Anechoic (may be transudate or exudate)
- Hypoechoic (often exudate)
- Complex septated or non-septated (always exudate)
- Homogeneously echogenic (always exudate) 2
- Reduces thoracentesis complications from 33-50% to nearly 0% 2
Diagnostic Thoracentesis
Essential Pleural Fluid Tests
- Nucleated cell count and differential
- Total protein
- Lactate dehydrogenase (LDH)
- Glucose
- pH
- Amylase
- Cytology 1
Additional Tests Based on Clinical Suspicion
- Gram stain and culture (if infectious etiology suspected)
- Adenosine deaminase (if tuberculosis suspected)
- Triglycerides (if chylothorax suspected)
- NT-proBNP (to differentiate cardiac from non-cardiac causes)
Interpretation of Results
Transudate vs. Exudate Differentiation
- Use Light's criteria to differentiate:
- Pleural fluid protein/serum protein ratio >0.5
- Pleural fluid LDH/serum LDH ratio >0.6
- Pleural fluid LDH >2/3 upper limit of normal serum LDH
Specific Diagnostic Criteria
Parapneumonic effusion/empyema:
- Simple: pH >7.2, LDH <1000 IU/L, glucose >2.2 mmol/L, negative cultures
- Complicated: pH <7.2, LDH >1000 IU/L, positive Gram stain or culture
- Empyema: Frank pus, usually positive cultures 2
Malignant effusion:
- Positive cytology
- CT findings: nodular pleural thickening, pleural masses, lung masses
Management Decision Algorithm
Small asymptomatic effusions (<10 mm thickness on ultrasound):
- Observation with follow-up imaging 2
Moderate symptomatic effusions:
- Therapeutic thoracentesis for symptom relief
- Treat underlying cause based on diagnostic results
Complicated parapneumonic effusions (pH <7.2):
- Chest tube drainage
- Consider tissue plasminogen activator/DNase therapy 2
Malignant effusions:
- Therapeutic thoracentesis for palliation in very short life expectancy
- Consider pleurodesis or indwelling pleural catheter for recurrent effusions 2
Common Pitfalls to Avoid
Failing to use ultrasound guidance for thoracentesis
- Increases risk of pneumothorax and unsuccessful sampling
Incomplete fluid analysis
- Missing critical diagnostic tests can lead to misdiagnosis
Delaying drainage of complicated parapneumonic effusions
- Can lead to empyema and increased morbidity/mortality
Overlooking malignancy
- Always send fluid for cytology, even when another cause seems likely
Not considering less common causes
- Pulmonary embolism, rheumatologic diseases, and tuberculosis can be missed without appropriate testing
By following this systematic approach to the workup of moderate pleural effusions, you can efficiently determine the underlying cause and implement appropriate treatment, ultimately improving patient outcomes and quality of life.