Measuring Pleural Effusion for Thoracentesis
Ultrasound should be used to measure and localize all pleural effusions prior to thoracentesis, with the maximal interpleural distance measured at the thoracic base during end-expiration providing the most accurate volume estimation. 1
Primary Measurement Technique
Use bedside ultrasound with the patient supine (trunk elevated 15 degrees) to measure the maximal distance between parietal and visceral pleura (interpleural distance) at the thoracic base in the posterior axillary line during end-expiration. 2, 3
Volume Estimation Formula
- For practical bedside estimation: Volume (mL) = 20 × interpleural distance (mm) 2
- An interpleural distance of 20 mm corresponds to approximately 380 mL (±130 mL) 4
- An interpleural distance of 40 mm corresponds to approximately 1,000 mL (±330 mL) 4
- An interpleural distance >45 mm (right side) or >50 mm (left side) predicts effusions ≥800 mL with 94-100% sensitivity 3
Measurement Accuracy
- Ultrasound correlates better with actual effusion volume (r = 0.80) compared to lateral decubitus radiography (r = 0.58) 4
- Mean prediction error with ultrasound is 224 mL versus 465 mL with radiography 4
- Ultrasound has 84% sensitivity, 100% specificity, and 94% accuracy for diagnosing pleural effusion 1
Ultrasound Guidance for Drainage
Ultrasound guidance should be used for all thoracentesis procedures, particularly for small or loculated effusions (Grade 1B recommendation). 1
Safety Benefits
- Complications (pneumothorax, failure to acquire fluid) decreased from 33-50% to 0% with ultrasound guidance 1
- Success rate of obtaining fluid is 97% even after failed landmark-based attempts 1, 5
- Complication rate of 1.3% with ultrasound guidance versus 6.5% with landmark technique 1
- Zero incidence of pneumothorax or bleeding when performed under ultrasound guidance 2
Characterizing Effusion Complexity
Evaluate for internal echoes, septations, and loculations to distinguish simple from complex effusions before drainage. 1, 6
Ultrasound Findings
- Anechoic (echo-free) appearance suggests transudative effusion 1, 6
- Presence of septations AND loculations has 98% specificity and 94% positive predictive value for exudative effusion 6
- Fibrin, septations, and loculations together have 99% specificity and 96% positive predictive value for exudative effusion 6
- Septations are better visualized on ultrasound than CT scan 1, 7
- Septation presence increases odds of complicated parapneumonic effusion/empyema 5.3-fold 6
- Loculation presence increases odds of complicated parapneumonic effusion/empyema 3.3-fold 6
When to Use CT Instead
Reserve contrast-enhanced CT for suspected malignant pleural disease or when delineating complex loculated effusions that are difficult to drain. 1, 7
CT Indications
- Suspected malignancy requiring pleural characterization (nodular thickening, mediastinal involvement) 1
- Perform CT before complete drainage, as pleural abnormalities are better visualized with fluid present 7
- Delineating size and position of loculated effusions when drainage is difficult 1, 7
Minimum Effusion Size for Safe Drainage
Do not attempt thoracentesis for effusions <1 cm thickness on lateral decubitus view or ultrasound measurement. 5
- Small effusions (<1 cm) require ultrasound guidance if drainage is clinically indicated 5
- Ultrasound can successfully sample effusions as small as those visible only on imaging 1
Common Pitfalls
- Do not rely on supine chest radiography alone—it underestimates pleural fluid volume and has only 52% agreement with ultrasound 3
- Do not use landmark technique for small or loculated effusions—ultrasound guidance is essential 1
- Measure at end-expiration, not end-inspiration—expiratory measurements correlate better with actual volume 2, 3
- Position probe in posterior axillary line at lung base—this location provides the most accurate measurement 2, 3