What are the sensitivity and specificity of lung ultrasound compared with chest radiograph and chest computed tomography for diagnosing pneumothorax, pleural effusion, and bacterial pneumonia in adult patients?

Lung Ultrasound Diagnostic Performance Compared to Chest X-Ray and CT

Lung ultrasound demonstrates superior sensitivity to chest radiography across all major thoracic pathologies while maintaining comparable or superior specificity, and achieves diagnostic accuracy similar to CT for pleural effusion, consolidation, and pneumothorax. 1

Pneumothorax Detection

For pneumothorax, lung ultrasound achieves pooled sensitivity of 89% and specificity of 99%, substantially outperforming chest radiography which demonstrates only 30-75% sensitivity. 1

Specific Performance Metrics:

• Lung ultrasound sensitivity: 75-100% depending on methodology and patient population 1, 2, 3
• Chest X-ray sensitivity: 0-52.7% in critically ill patients 2, 3
• Lung ultrasound specificity: 93-100% 1, 2, 3
• Chest X-ray specificity: 99-100% 2, 3

The largest ICU series (357 hemithoraces) demonstrated 100% sensitivity for loss of lung sliding and 100% specificity for presence of a lung point when compared to CT. 1 A prospective study of 192 critically ill patients showed lung ultrasound sensitivity of 86.1% versus chest radiography's 52.7%, with diagnostic accuracy of 95.3% versus 90.6% respectively. 1, 3

Critical Technical Points:

• Diagnosis requires loss of lung sliding AND absence of B-lines AND absence of lung pulse 1
• Presence of a lung point confirms pneumothorax with near-100% specificity 1
• Visualization of B-lines reliably excludes pneumothorax 1
• Absence of lung sliding alone is insufficient—can occur with atelectasis, consolidation, or contusion 1

Important Limitation:

CT detects small "occult" pneumothoraces that both ultrasound and chest radiography miss, though clinical significance of these findings remains debatable. 1

Pleural Effusion Detection

Lung ultrasound achieves 100% sensitivity and 100% specificity for pleural effusion detection, clearly superior to supine chest radiography and equivalent to CT. 1, 2

Comparative Performance:

• Lung ultrasound: 100% sensitivity, 100% specificity 2
• Chest X-ray: 65% sensitivity, 81% specificity 2
• Lung ultrasound is more accurate than supine radiography and as accurate as CT 1

Lung ultrasound is superior to chest radiography in distinguishing between effusion and consolidation when opacities are identified. 1 The optimal scanning site is the posterior axillary line above the diaphragm using a microconvex transducer. 1

Pneumonia and Consolidation

Lung ultrasound demonstrates sensitivity and specificity exceeding 90% for alveolar consolidation, substantially outperforming chest radiography. 1

Specific Performance Data:

• Lung ultrasound: 100% sensitivity, 78% specificity, 95% diagnostic accuracy for consolidation 2
• Chest X-ray: 38% sensitivity, 89% specificity, 49% diagnostic accuracy for consolidation 2
• Lung ultrasound is as accurate as chest radiography in pediatric pneumonia diagnosis 1

The Bedside Lung Ultrasound in Emergency (BLUE) protocol achieves diagnostic accuracy exceeding 90% for the most common etiologies of acute respiratory failure. 1

Critical Caveat:

Central pneumonias are difficult to detect with both chest X-ray and lung ultrasound due to intervening aerated lung—this represents a significant limitation where CT may be necessary. 4

Interstitial Syndrome

Lung ultrasound demonstrates 94% sensitivity and 93% specificity for interstitial syndrome, compared to chest radiography's 46% sensitivity and 80% specificity. 2

Meta-Analysis Findings:

A meta-analysis comparing all three modalities in 543 critically ill patients with respiratory symptoms found:

• Chest X-ray: 49% sensitivity (95% CI 40-58%), 92% specificity (86-95%) 1
• Lung ultrasound: 95% sensitivity (92-96%), 94% specificity (90-97%) 1

Clinical Decision Algorithm

When to Use Lung Ultrasound as Primary Modality:

• ICU patients with respiratory failure (suggested as primary diagnostic modality) 1, 4
• Suspected pneumothorax in hemodynamically unstable patients 1
• Suspected pleural effusion requiring thoracentesis guidance 1, 4
• Bedside evaluation when patient transport poses risk 4
• Monitoring treatment response (repeatable without cumulative harm) 4

When Chest X-Ray Remains Appropriate:

• Routine post-procedural imaging (endotracheal intubation, chest tube, central line placement) 1
• When ultrasound expertise unavailable 4
• Complementary imaging when lung ultrasound findings equivocal 4

When CT is Mandatory:

• Immunocompromised patients with suspected pulmonary infection (lung ultrasound insufficient to rule out parenchymal disease) 4
• High clinical suspicion despite negative/indeterminate chest X-ray and lung ultrasound 4
• Assessment of disease severity and multilobar involvement 4
• Suspected complications requiring detailed anatomic evaluation 4
• Detection of small occult pneumothoraces 1

Key Advantages of Lung Ultrasound

• No radiation exposure 4
• No patient transport required 4
• Immediate bedside availability with real-time interpretation 4
• Lower cost than CT 4
• Repeatable for monitoring without cumulative harm 4
• Can be performed during cardiac arrest scenarios 1

Critical Limitations to Recognize

• Mechanically ventilated patients have limited evaluation 4
• Complete lung visualization impossible (unlike CT) 4
• Small occult pneumothoraces may be missed 4
• Central pneumonias difficult to detect 4
• Operator competence and experience significantly impact accuracy 1
• Subcutaneous emphysema can obscure findings 5