Which diagnostic tests should be used to rule in or rule out a pneumothorax?

Diagnostic Testing for Pneumothorax

A standard upright posteroanterior (PA) chest radiograph is the initial diagnostic test of choice to rule in or rule out pneumothorax, with lung ultrasound serving as a superior alternative when immediately available, particularly in critical care settings. 1

Primary Diagnostic Approach

First-Line Imaging

Plain chest radiography (PA view) establishes the diagnosis in both primary and secondary spontaneous pneumothorax. 1 This remains the standard initial test, though it has important limitations:

• Expiratory films are NOT recommended - they add little diagnostic value and should not be performed routinely, even for suspected small apical pneumothoraces 1, 2
• Standard inspiratory PA radiographs are sufficient for initial screening
• Supine chest radiographs have poor sensitivity (only 50.2% sensitive) and should be avoided when possible 3

Lung Ultrasound: The Superior Alternative

When available, lung ultrasound outperforms chest radiography and should be strongly considered as the initial diagnostic test. 4, 3 The evidence is compelling:

• 90.9% sensitive and 98.2% specific for pneumothorax detection 3
• Dramatically superior to supine chest radiography (which is only 50.2% sensitive) 3
• In trauma patients, ultrasound achieves 68.4% sensitivity versus only 23.5% for chest radiography 5
• More accurate than supine radiography in ruling out pneumothorax 4

Ultrasound Technique

The diagnosis relies on identifying four key sonographic signs in sequence 4:

1. Absence of lung sliding (rhythmic movement between pleural layers)
2. Absence of B-lines (their presence excludes pneumothorax at that location)
3. Absence of lung pulse (cardiac oscillations at the pleural line)
4. Presence of lung point (transition zone between pneumothorax and normal lung - confirms diagnosis and helps determine size)

In extreme emergencies, the absence of any pleural line movement (sliding or pulse) plus absence of B-lines allows immediate pneumothorax diagnosis without searching for the lung point. 4

When Initial Imaging is Inconclusive

Additional Plain Film Views

If pneumothorax is suspected but not confirmed on standard PA radiograph 1:

• Lateral radiographs provide additional information in up to 14% of cases 1
• Lateral decubitus radiographs are superior to erect or supine films and considered as sensitive as CT scanning for pneumothorax detection 1
• These additional views are particularly valuable in secondary pneumothorax patients where even small pneumothoraces have significant clinical implications

CT Scanning Indications

CT scanning should be reserved for specific difficult scenarios 1:

• Severe bullous lung disease - differentiates emphysematous bullae from true pneumothorax, preventing unnecessary and potentially dangerous aspiration 1
• Lungs obscured by surgical emphysema 1
• Suspected complex cystic lung disease requiring differentiation from bullae 1
• When exact size quantification is required - CT is the most robust method 1

Routine preoperative CT in patients with suspected emphysema or isolated bullae adds little to management beyond the PA chest radiograph. 1

Tests That Should NOT Be Performed

• Pulmonary function tests - weakly sensitive and not recommended 1
• Routine expiratory radiographs - add minimal diagnostic value 1, 2
• Arterial blood gases - frequently abnormal but not diagnostic; do not reliably indicate pneumothorax presence or size 1

Clinical Context Matters

Clinical history and physical examination suggest pneumothorax presence but are unreliable indicators of size. 1 Key clinical pearls:

• Secondary pneumothorax patients typically have breathlessness out of proportion to radiographic size 1
• 46% of primary pneumothorax patients wait more than 2 days before seeking care 1
• Breathless patients require intervention regardless of radiographic pneumothorax size 1

Practical Algorithm

1. Stable patient, suspected pneumothorax: Upright PA chest radiograph
2. Critical/unstable patient or trauma: Lung ultrasound (if operator available) or upright PA radiograph
3. PA radiograph negative but high clinical suspicion: Add lateral or lateral decubitus view, or perform lung ultrasound
4. Complex bullous disease or surgical emphysema obscuring lungs: CT chest
5. Cardiac arrest/extreme emergency with ultrasound available: Immediate lung ultrasound at bedside

The key advantage of ultrasound is immediate bedside availability in critical situations, superior sensitivity compared to portable radiography, and the ability to perform it during initial trauma assessment without additional equipment or time 4, 3, 6.