Can a chest x-ray (CXR) miss a pleural effusion?

Can a Chest X-ray Miss a Pleural Effusion?

Yes, chest X-rays can definitely miss pleural effusions, particularly small ones under 200 mL, subpulmonic effusions, or those obscured by adjacent pathology. 1

Detection Limitations of Chest X-rays

Chest radiography has significant limitations in detecting pleural fluid:

• Volume thresholds: According to the ACR Appropriateness Criteria, chest X-rays typically require:

  • 175 mL of fluid to be visible on frontal (PA) views

  • 75 mL of fluid to be visible on lateral views 1

  • As little as 50 mL can produce detectable posterior costophrenic angle blunting on lateral radiographs 1

• Patient positioning matters significantly:

  • Supine radiographs often underestimate pleural fluid volume 1
  • In ICU settings with supine patients, free fluid layers posteriorly, creating only subtle signs like hazy opacity with preserved vascular shadows 1

• Easily missed presentations:

  • Subpulmonic effusions (fluid accumulating beneath the lung) are particularly challenging to detect on PA radiographs 1
  • Loculated effusions may be missed without specialized views 1
  • Effusions adjacent to consolidations are frequently missed 1

Superior Alternatives for Detecting Small Effusions

Ultrasound

• Can detect as little as >20 mL of pleural fluid 1
• Has higher sensitivity than chest X-ray for detecting small effusions 1
• European Society of Intensive Care Medicine strongly recommends ultrasound for pleural effusion detection 1
• Provides additional information about effusion characteristics (septations, echogenicity) 1
• Shows 92% positive predictive value compared to lateral decubitus radiography 2

CT Scan

• Can detect as little as >10 mL of pleural fluid 1
• Considered the reference standard for imaging pleural effusions 1
• Provides detailed information about adjacent structures and underlying pathology 1

Improving Detection on Chest X-ray

To maximize detection of pleural effusions on chest X-ray:

1. Use optimal positioning:

   • Lateral decubitus views significantly improve detection of free-flowing effusions 1
   • Oblique semi-supine position may be better for encapsulated fluid and more comfortable for patients 3

2. Look for subtle signs on supine films:

   • Increased density of the hemithorax
   • Blunted costophrenic angle
   • Loss of hemidiaphragm silhouette 4

3. Consider specialized views for suspected subpulmonic effusions:

   • Lateral decubitus view
   • Ultrasound examination 1

Clinical Implications

The limitations of chest X-rays in detecting pleural effusions have important clinical consequences:

• Small but clinically significant effusions may be missed, delaying diagnosis
• Ultrasound should be considered when there's clinical suspicion despite negative chest X-ray
• For critically ill patients, supine chest X-rays are particularly unreliable for effusion detection
• When accurate assessment of pleural fluid is crucial for management decisions, ultrasound or CT should be considered

In summary, while chest X-rays remain a useful first-line imaging modality, their limitations in detecting small, loculated, or subpulmonic effusions must be recognized, and additional imaging with ultrasound or CT should be considered when clinical suspicion remains despite negative radiographs.