What is the minimum amount of pleural effusion that can be detected with Ultrasound Scan (USS)?

Minimum Volume of Pleural Effusion Detectable by Ultrasound

Ultrasound can detect pleural effusions as small as 20 mL, making it significantly more sensitive than conventional radiography for detecting small pleural collections. 1

Comparative Detection Thresholds by Imaging Modality

Different imaging modalities have varying sensitivity for detecting pleural effusions:

• Ultrasound: Detects as little as 20 mL of pleural fluid 1
• Chest CT: Detects >10 mL of pleural fluid (reference standard) 1
• Chest X-ray:
  • Lateral view: Requires >75 mL 1
  • Frontal (PA) view: Requires >175 mL 1
  • Posterior costophrenic angle blunting on lateral: Requires approximately 50 mL 1

Ultrasound Advantages for Pleural Effusion Detection

Ultrasound offers several advantages for pleural effusion detection:

• High sensitivity and specificity: 84% sensitivity, 100% specificity, and 94% accuracy for diagnosing pleural effusions 1
• Real-time assessment: Allows immediate visualization and guidance for procedures 1
• No radiation exposure: Safe for repeated examinations 2
• Portability: Can be performed at bedside with patient sitting or recumbent 1
• Superior characterization: Better visualization of fibrinous septations compared to CT scans 1, 2

Ultrasound Characteristics of Pleural Effusions

Pleural effusions appear as anechoic (black) or hypoechoic (dark with internal echoes) regions above the diaphragm on ultrasound:

• Anechoic fluid: May be either transudate or exudate 1
• Complex septated, complex non-septated, or homogeneously echogenic patterns: Always exudates 1

Clinical Applications

Ultrasound has important clinical applications for pleural effusions:

• Diagnostic guidance: Recommended for complementing physical examination and conventional chest radiography 1
• Procedural guidance: Should be used to assist in drainage, particularly for small or loculated effusions 1
  • Increases success rate to 97% in loculated effusions 1
  • Reduces complications from 33-50% to 0% when draining large pleural effusions 1
• Volume estimation: Can accurately quantify pleural fluid volume 1
  • A simplified formula: Volume (mL) = 20 × Separation (mm) between parietal and visceral pleura 3

Practical Considerations

When performing ultrasound for pleural effusion:

• Position the probe in the posterior axillary line and obtain transverse sections perpendicular to the body axis 3
• Measure the maximal distance between parietal and visceral pleura at end-expiration 3
• For diagnostic thoracentesis, at least 60 mL should be aspirated when using direct smear/cytospin, and ≥150 mL when both direct smear/cytospin and cell block preparations are used 4

Safety Considerations

• Ultrasound-guided thoracentesis has an excellent safety profile with minimal complications 1
• Large-volume thoracentesis (>1L) is generally safe with a very low risk (0.5%) of clinical reexpansion pulmonary edema 5
• Appropriately trained clinicians can perform ultrasound-guided drainage with an acceptable complication profile 1

In summary, ultrasound is the most sensitive non-invasive imaging modality for detecting small pleural effusions, capable of visualizing as little as 20 mL of fluid, and should be the preferred initial imaging method when small effusions are suspected.