How can a pleural effusion be identified on chest radiograph and what are the recommended diagnostic and therapeutic steps?

Identifying Pleural Effusion on Chest Radiograph

Ultrasound must be used to confirm the presence of pleural fluid collection, as it is the most accurate and direct test for detecting pleural effusion and should be performed in all suspected cases. 1

Radiographic Signs on Chest X-Ray

Upright Posteroanterior/Anteroposterior Films

• Obliteration of the costophrenic angle is the earliest radiographic sign of pleural effusion, requiring approximately 200 mL of fluid to become visible 2, 1
• A meniscus sign (rim of fluid ascending the lateral chest wall) appears on upright films and correlates with roughly 100 mL when below the hemidiaphragm, 250 mL at the level of the hemidiaphragm, and 650 mL when obscuring the hemidiaphragm 2, 3
• Homogeneous increased opacity over the hemithorax, including layering and gradient opacities, is the most sensitive sign of effusion 4
• Large effusions may produce complete "white out" of the hemithorax, making it impossible to differentiate from severe consolidation or collapse without ultrasound 2, 1

Lateral Chest Radiographs

• Lateral films are more sensitive than PA views, detecting as little as 50 mL of fluid by blunting of the posterior costophrenic angle (mean 26 mL) 1, 3
• However, routine lateral radiographs are not recommended in the initial workup, as they rarely add clinically significant information beyond what ultrasound provides 2

Supine Radiographs (Critical Pitfall)

• Supine films significantly underestimate pleural fluid because fluid layers posteriorly, creating a homogeneous increase in opacity over the entire lung field without classic costophrenic angle blunting 1, 5
• Supine radiographs have only 67% sensitivity and 70% specificity for detecting effusions, with 175-525 mL required to produce noticeable density changes 5, 6
• A normal supine radiograph does not exclude pleural effusion 5

Diagnostic Algorithm

Step 1: Initial Imaging Assessment

• Obtain upright PA chest radiograph as the initial screening test 2
• If pleural effusion is suspected clinically but not clearly visible, or if the patient cannot stand, proceed immediately to ultrasound rather than obtaining additional plain films 1

Step 2: Ultrasound Confirmation (Mandatory)

• Portable bedside ultrasound must be performed to confirm fluid presence, estimate volume, and differentiate free from loculated fluid 2, 1
• Ultrasound detects effusions missed on standard radiography and visualizes fibrinous septations better than CT 1
• Ultrasound should guide any subsequent thoracentesis or drain placement 2

Step 3: Diagnostic Thoracentesis

When effusion is confirmed and etiology is unclear:

• Always use ultrasound guidance to reduce complication risk 7
• Obtain 25-50 mL of pleural fluid for optimal diagnostic yield 7
• Send fluid for:
  • Cell count with differential
  • Biochemistry (protein, LDH, glucose, pH if parapneumonic effusion suspected) 2
  • Cytology (both direct smear and cell block) if malignancy suspected 7
  • Culture (5-10 mL in aerobic and anaerobic blood culture bottles plus routine culture) if infection possible 2, 7
• Limit drainage to 1-1.5 L per session to prevent re-expansion pulmonary edema 8, 7

Step 4: Classification and Further Management

For parapneumonic effusions/empyema:

• Simple parapneumonic (pH >7.2, LDH <1000 IU/L, glucose >2.2 mmol/L, clear fluid): Treat with antibiotics alone 2
• Complicated parapneumonic (pH <7.2, LDH >1000 IU/L): Requires chest tube drainage 2
• Empyema (frank pus): Requires chest tube drainage; do not measure pH on grossly purulent fluid 2

For suspected malignant effusion:

• If initial cytology is negative but suspicion remains high, proceed directly to pleural biopsy (medical thoracoscopy preferred, >92% sensitivity) rather than repeat thoracentesis 7

Step 5: CT Scanning (Selective Use)

• Do not perform CT routinely for pleural effusion assessment 1
• Reserve contrast-enhanced CT for:
  • Differentiating empyema from lung abscess (look for "split pleura sign" and lenticular shape) 2
  • Evaluating difficult-to-drain loculated effusions 1
  • Assessing pleural thickening when malignancy is suspected 7
• If CT is obtained, perform it before complete drainage, as pleural abnormalities are better visualized with fluid present 1

Common Pitfalls to Avoid

• Never rely on chest radiograph alone to exclude pleural effusion—66% of effusions are missed on plain films, with only 53% of small and 71% of moderate effusions detected 4
• Do not assume supine films are adequate—they miss most small-to-moderate effusions 1, 5
• Radiographs cannot differentiate empyema from simple parapneumonic effusion—biochemical analysis is required 2
• In children presenting with "white out," ultrasound is essential to distinguish large effusion from severe consolidation/collapse 2