How to differentiate between empyema and lung abscess on a computed tomography (CT) scan in a patient with suspected pleural effusion and compromised immune system?

CT Differentiation of Empyema vs Lung Abscess

Contrast-enhanced CT is the gold standard for distinguishing empyema from lung abscess, and this distinction is critical because empyema requires drainage while lung abscess drainage can be life-threatening. 1, 2, 3

Key CT Features to Distinguish These Entities

Empyema Characteristics on CT:

• Lenticular (lens-shaped) configuration that conforms to the pleural space 1, 2
• "Split pleura sign" - enhancement of both visceral and parietal pleura with fluid between them 1, 2
• Compresses adjacent lung parenchyma rather than destroying it 1
• Pleural thickening and enhancement with IV contrast 1, 2, 3
• Loculations and septations within the collection 1, 2
• Extrapleural fat stranding 1, 2
• Smooth, well-defined margins 1

Lung Abscess Characteristics on CT:

• Spherical or round shape within the lung parenchyma 2, 4
• Indistinct boundary between the collection and surrounding lung tissue 1, 2
• Thick enhancing wall with central necrosis on contrast-enhanced images 1, 2, 3
• Located within lung tissue, not the pleural space 2, 4
• May contain air-fluid levels 4, 5
• Destroys rather than compresses lung parenchyma 1

Why IV Contrast is Essential

CT with IV contrast is mandatory for accurate diagnosis - non-contrast CT has insufficient evidence for evaluating pulmonary abscess and cannot adequately characterize pleural enhancement patterns. 1, 3 The contrast enhancement allows visualization of:

• Pleural enhancement in empyema (the split pleura sign) 1, 2, 3
• Thick wall enhancement in lung abscess 1, 3
• Degree of loculation and septation 1, 3

Critical Clinical Implications

Treatment Differs Dramatically:

• Empyema requires chest tube drainage plus antibiotics - antibiotics alone are insufficient 1, 2
• Lung abscess requires antibiotics alone - drainage is contraindicated as first-line therapy and can cause life-threatening complications 2, 4, 5

Special Scenario - Coexisting Conditions:

When empyema and lung abscess coexist, drain only the empyema - the antibiotics given for empyema will simultaneously treat the lung abscess. 1, 2 Never drain the lung abscess component. 1, 2

Role of Ultrasound in Immunocompromised Patients

While CT with contrast remains the gold standard, ultrasound demonstrates 94-96% sensitivity and 96-100% specificity for differentiating empyema from lung abscess and can be particularly useful for guiding drainage procedures. 1, 3 Ultrasound shows:

• Echogenic effusions suggesting exudates or empyema 1
• Septations within complex effusions 1, 3
• Real-time guidance for thoracentesis 3

However, ultrasound has limitations in obese patients, those with subcutaneous emphysema, and cannot visualize centrally located lesions. 1

Common Pitfalls to Avoid

• Never drain a lung abscess as initial therapy - this can cause bronchopleural fistula, empyema, or massive hemoptysis 2, 4
• Do not use CTA chest protocols when evaluating for empyema/abscess, as these are suboptimal for pleural enhancement assessment 3
• Do not rely on chest radiography alone - overlying lung disease or unusual location can make differentiation impossible 1, 4
• In immunocompromised patients with suspected pleural infection, perform diagnostic thoracentesis on all parapneumonic effusions to guide management 1, 5

Prognostic Considerations

Patients with both empyema and lung abscess have significantly higher mortality (odds ratio 4.685) and ICU admission rates compared to empyema alone, particularly in elderly patients with multiple comorbidities. 6, 7 This underscores the importance of accurate CT diagnosis to guide aggressive early management.