How does a pleural effusion appear on a chest X-ray (Chest X-ray)?

Radiographic Appearance of Pleural Effusion on Chest X-Ray

On a standard PA chest radiograph, pleural effusion appears as blunting of the costophrenic angle when approximately 200 mL of fluid is present, while lateral chest radiographs can detect as little as 50 mL of fluid by showing posterior costophrenic angle blunting. 1

Standard Upright PA and Lateral Chest Radiograph Findings

Volume Detection Thresholds

• PA chest radiograph becomes abnormal with approximately 200 mL of pleural fluid 1
• Lateral chest radiograph can detect as little as 50 mL of fluid by showing posterior costophrenic angle blunting 1, 2
• The meniscus sign becomes visible on lateral radiograph at approximately 50 mL, and on PA radiograph at approximately 200 mL 2
• At approximately 500 mL, the meniscus obscures the hemidiaphragm 2

Classic Radiographic Appearances

• Homogeneous opacity with a meniscus is the most sensitive sign of pleural effusion 3
• The fluid creates a characteristic curved upper border (meniscus) that is higher laterally than medially 1
• Blunting of the costophrenic angle is the cardinal finding on PA radiograph 1
• Moderate to large effusions (500-2,000 mL) occupy one-third to two-thirds of the hemithorax 4

Supine Chest Radiograph Findings (ICU Setting)

In supine positioning, pleural effusion appears dramatically different and is often underestimated:

• Hazy opacity of one hemithorax with preserved vascular shadows is the characteristic appearance 1
• Free pleural fluid layers posteriorly rather than dependently 1
• Loss of the sharp silhouette of the ipsilateral hemidiaphragm 1
• Thickening of the minor fissure may be visible 1
• The supine chest radiograph frequently underestimates the actual volume of pleural fluid 1, 5

Important Caveat

Bedside chest radiographs in ICU patients detect large pleural effusions 92% of the time, but small and medium effusions are often misdiagnosed as parenchymal opacities (45%) or missed entirely (55%) 3

Subpulmonic Effusion Appearance

Subpulmonic effusions present a diagnostic challenge with atypical features:

• Lateral peaking of an apparently raised hemidiaphragm with a steep lateral slope and gradual medial slope on PA radiograph 1
• On lateral radiograph, shows a flat appearance of the posterior aspect of the hemidiaphragm with a steep downward slope at the major fissure 1
• These are often transudates and may require lateral decubitus views or ultrasound for confirmation 1

Lateral Decubitus Films

• Lateral decubitus films demonstrate free fluid gravitating to the most dependent part of the chest wall, effectively differentiating between pleural thickening and free fluid 1, 4
• This technique is particularly useful when subpulmonic effusion is suspected 1

Size Classification by Radiographic Appearance

• Small effusions (<500 mL): Occupy less than one-third of the hemithorax 4
• Moderate effusions (500-1,500 mL): Occupy approximately one-third to two-thirds of the hemithorax 4
• Large effusions: Occupy more than two-thirds of the hemithorax 4
• Massive effusions: Occupy the entire hemithorax (malignancy is the most common cause) 1

Critical Diagnostic Pitfalls to Avoid

• Supine radiographs systematically underestimate pleural fluid volume because fluid layers posteriorly and appears as diffuse haziness rather than a discrete collection 1, 5
• Atelectasis can occasionally mimic the pleural veil sign of effusion, accounting for most false-positive findings 3
• Absence of contralateral mediastinal shift in large effusions implies mediastinal fixation, mainstem bronchus occlusion by tumor, or extensive pleural involvement (as in mesothelioma) 1
• Loculated effusions produce irregular radiographic appearances that may not follow typical patterns 4

When Chest X-Ray Is Insufficient

Ultrasound should be used when:

• The chest radiograph shows "white out" where solid consolidation cannot be differentiated from large effusion 5
• Small or loculated effusions are suspected but not clearly visible 1, 5
• More accurate volume estimation is needed (ultrasound can detect as little as 20 mL) 4
• Guidance for thoracentesis is required 1, 5