BLUE Protocol for Lung Point-of-Care Ultrasound
The BLUE (Bedside Lung Ultrasound in Emergency) protocol is a rapid, systematic ultrasound examination that evaluates three anterior chest points bilaterally plus lateral points to diagnose the cause of acute respiratory failure in under 3 minutes, achieving >90% diagnostic accuracy for differentiating between pulmonary edema, pneumonia, COPD/asthma, pulmonary embolism, and pneumothorax. 1, 2
Core Technique and Scanning Points
The protocol uses a microconvex probe to examine standardized chest locations 1:
- Three anterior points per hemithorax: Upper and lower BLUE points along the midclavicular line
- Posterolateral alveolar and/or pleural syndrome (PLAPS) point: Evaluated when anterior findings are inconclusive
- Venous analysis: Compression ultrasound of femoral veins when pulmonary embolism is suspected
Essential Ultrasound Signs (10 Key Findings)
You must recognize these artifacts to interpret the protocol 1, 2:
Normal lung surface:
- Bat sign: Two ribs with acoustic shadow and pleural line between them
- Lung sliding: Shimmering movement of pleural line with respiration (produces "seashore sign" on M-mode)
- A-lines: Horizontal repetitive artifacts parallel to pleural line
Pathological findings:
- B-lines (lung rockets): Vertical laser-like artifacts extending from pleura to screen edge, indicating interstitial syndrome
- Lung consolidation: Tissue-like appearance (fractal sign) replacing normal aeration
- Pleural effusion: Anechoic space between parietal and visceral pleura (quad sign, sinusoid sign)
- Pneumothorax indicators: Absent lung sliding with A-lines (stratosphere sign on M-mode) and lung point (transition between sliding and non-sliding lung—pathognomonic for pneumothorax)
Diagnostic Profiles and Their Meanings
The protocol assigns specific profiles based on bilateral anterior findings 1, 3:
A Profile (Predominant A-lines + lung sliding bilaterally)
- Diagnosis: COPD or asthma
- Accuracy: 85% sensitivity, 89% specificity 3
- Caveat: If venous thrombosis detected → pulmonary embolism instead
A' Profile (A-lines WITHOUT lung sliding)
- Diagnosis: Pneumothorax (confirm with lung point)
- Accuracy: 80-100% sensitivity, 100% specificity 3
B Profile (Bilateral anterior B-lines/lung rockets + lung sliding)
- Diagnosis: Cardiogenic pulmonary edema
- Accuracy: 92% sensitivity, 100% specificity 3
- Mechanism: B-lines correlate with pulmonary artery occlusion pressure >18 mmHg 1
A/B Profile (A-lines on one side, B-lines on other)
- Diagnosis: Pneumonia
- Confirm with: PLAPS point showing consolidation or effusion posterolaterally
- Accuracy: 94% sensitivity, 94% specificity 3
C Profile (Anterior lung consolidation)
- Diagnosis: Pneumonia
- Distinguish from atelectasis: Look for dynamic air bronchogram (rules in pneumonia) vs. lung pulse (suggests atelectasis) 2
Clinical Integration and Workflow
When to use: Any patient presenting with acute dyspnea or respiratory distress 4
Advantages over standard workup:
- POCUS added to standard diagnostic pathways significantly increases correct diagnoses compared to clinical examination alone 4
- No radiation exposure (critical for repeated assessments and pediatric/neonatal patients) 5
- Performed in real-time by treating clinician, enabling immediate management decisions 4
Performance characteristics: The BLUE protocol demonstrates diagnostic accuracy equivalent to CT scanning for acute respiratory failure 3, with sensitivities and specificities consistently >90% for major causes of dyspnea 5
Critical Pitfalls to Avoid
Subcutaneous emphysema or large body habitus: May obscure lung sliding—do not diagnose pneumothorax on absent sliding alone; always seek the lung point 1
Pneumonia vs. atelectasis confusion: Both show consolidation, but pneumonia has dynamic air bronchogram (moving with respiration), while atelectasis shows lung pulse (cardiac oscillations transmitted through airless lung) 2
Focal vs. diffuse B-lines: Focal B-lines suggest pneumonia; diffuse bilateral B-lines indicate pulmonary edema 5
Consolidations not reaching pleura: Lung ultrasound cannot detect central lung pathology that doesn't contact the pleural surface 5, 6
Indeterminate results: Studies rarely report the proportion of inconclusive scans—when findings don't fit a clear profile, proceed to CT imaging 4
Monitoring and Therapeutic Applications
Beyond diagnosis, the BLUE protocol enables real-time monitoring 5:
- Quantify pulmonary edema severity: Count total B-lines (28-point technique) or number of positive scanning sites (8-region technique) 5
- Track treatment response: Decreasing B-lines indicate successful diuresis in heart failure 5
- Guide fluid resuscitation: In the FALLS protocol (Fluid Administration Limited by Lung Sonography), appearance of B-lines serves as endpoint for fluid therapy, preventing iatrogenic pulmonary edema 1, 2
Evidence Quality Considerations
The 2021 American College of Physicians guideline 4 found that while POCUS improves diagnostic accuracy, in-hospital mortality and length of stay did not differ significantly between patients who did or did not receive POCUS—likely because the studies assessed diagnostic accuracy rather than patient-centered outcomes. However, the 2012 international consensus recommendations [5-5] provide strong Level A evidence supporting lung ultrasound for detecting consolidation, differentiating causes of dyspnea, and monitoring therapy response.