How do you interpret a chest x-ray (CXR)?

How to Interpret a Chest X-Ray

A systematic approach to chest X-ray interpretation requires evaluating technical quality first, followed by a structured assessment of airways, bones, cardiac silhouette, diaphragm, lung fields, mediastinum, and pleural spaces—though chest X-rays have significant limitations with sensitivity as low as 28% for detecting pulmonary metastases and may be normal early in diseases like pneumonia. 1, 2

Technical Quality Assessment

Before interpreting any findings, verify the technical adequacy of the image:

• Check for proper patient positioning by ensuring the medial ends of the clavicles are equidistant from the spinous processes (rotation assessment) 3
• Assess penetration by confirming vertebral bodies are just visible through the cardiac silhouette 3
• Verify inspiration by counting posterior ribs (should see 9-10 posterior ribs or 5-6 anterior ribs above the diaphragm) 4
• Confirm the image includes the lung apices superiorly and costophrenic angles inferiorly 4

Systematic Review Approach

Airways and Trachea

• Evaluate tracheal position for deviation, which may indicate mass effect, pneumothorax, or atelectasis 3
• Assess the carina at approximately T4-T5 level for widening that might suggest lymphadenopathy 3

Bones and Soft Tissues

• Examine all visible bones including ribs, clavicles, scapulae, and visible spine for fractures, lytic or blastic lesions 3
• Review soft tissues for subcutaneous emphysema or masses 4
• Check for medical devices if present, verifying proper positioning of endotracheal tubes, central lines, chest tubes, or pacemakers 3

Cardiac Silhouette

• Measure the cardiothoracic ratio on a PA film (should be <0.5 in adults; cardiomegaly if >0.5) 1
• Evaluate cardiac borders for clarity—loss of border definition suggests adjacent lung pathology 4
• Assess for mediastinal widening which may indicate aortic pathology, lymphadenopathy, or masses 1

Diaphragm

• Confirm normal hemidiaphragm position (right typically 1-2 cm higher than left) 4
• Look for free air under the diaphragm indicating pneumoperitoneum 5
• Assess costophrenic angles for blunting suggesting pleural effusion 5

Lung Fields

The lung parenchyma requires the most detailed scrutiny, as chest X-rays have substantial limitations—they may be normal in up to 64% of early pneumonia cases and miss small peripheral nodules entirely. 2

• Compare both lungs systematically from apex to base, looking for asymmetry 4
• Identify consolidation (air-space opacification with air bronchograms) which may represent pneumonia, though absence doesn't exclude it 2, 5
• Search for nodules or masses, recognizing that chest X-ray sensitivity is poor compared to CT 1
• Detect pneumothorax by looking for visceral pleural line and absence of lung markings peripherally 5
• Assess for interstitial patterns including reticular, nodular, or reticulonodular patterns suggesting fibrosis or infection 5, 6
• Evaluate for atelectasis indicated by volume loss, displaced fissures, or crowded vessels 5

Mediastinum

• Examine mediastinal contours for masses or lymphadenopathy, though chest X-ray has limited sensitivity 1
• Assess the aortic knob for calcification, tortuosity, or aneurysmal dilatation 1

Pleural Spaces

• Look for pleural effusions starting at costophrenic angles (blunting requires ~200-300 mL fluid) 5
• Identify pleural thickening or calcification suggesting prior inflammation or asbestos exposure 5

Critical Limitations to Recognize

Chest X-rays have substantial diagnostic limitations that must be acknowledged:

• Sensitivity for pulmonary metastases is only 28% compared to chest CT, making it inadequate for cancer staging 1
• Early pneumonia may show normal X-ray findings in up to 64% of cases; consider repeating in 2 days if clinical suspicion remains high 2
• Small peripheral nodules are frequently missed, particularly those <1 cm 1
• Observer variability is substantial with significant within- and between-reader disagreement 1

When Chest X-Ray Is Insufficient

If clinical suspicion for serious pathology remains despite normal chest X-ray, advanced imaging is mandatory:

• Order chest CT for suspected malignancy, pulmonary embolism, or when metastatic disease screening is needed 1
• Consider lung ultrasound which has 93-96% sensitivity and specificity for pneumonia, superior to chest X-ray 2
• Repeat chest X-ray in 48 hours if pneumonia is suspected clinically but initial film is negative 2
• Use elevated CRP (>100 mg/L) to support pneumonia diagnosis when imaging is negative or equivocal 2

Emerging AI-Assisted Interpretation

Artificial intelligence tools are increasingly available for chest X-ray interpretation, with CE-marked software showing pooled sensitivity of 98.57% and specificity of 98.05% for tuberculosis detection, though human oversight remains essential. 1, 7

• AI algorithms can assist with detection of nodules, consolidation, pneumothorax, pleural effusion, and cardiomegaly 5, 7
• Deep learning approaches show more stable performance than traditional machine learning methods 1
• AI tools are particularly valuable in resource-limited settings and for tuberculosis screening 1, 7
• Human radiologist review remains mandatory as AI has limitations with data quality and interpretability 6, 7