Diagnostic Method for Empyema
Chest radiography should be performed as the initial imaging study, followed by ultrasound to confirm pleural fluid and guide thoracentesis, with diagnostic pleural fluid analysis (including pH, glucose, LDH, Gram stain, and culture) being essential to establish the diagnosis of empyema. 1, 2
Initial Clinical Evaluation
Suspect empyema in patients with pneumonia who fail to respond to appropriate antibiotic therapy, presenting with fever, chest pain, and breathlessness. 1
Imaging Algorithm
Step 1: Chest Radiography
- Obtain posteroanterior and lateral chest X-rays as the initial imaging study to detect pleural effusion and assess for pulmonary infiltrates. 1, 2
- PA and lateral views have significantly greater sensitivity (83.9%) compared to single-view AP radiographs (67.3%) for detecting pleural effusions. 2
- Be aware that most missed parapneumonic effusions occur in patients with coexistent lower lobe consolidation. 2
Step 2: Ultrasound (Preferred Imaging Modality)
- Ultrasound must be performed to confirm the presence of pleural fluid collection and guide further management. 1, 2
- Ultrasound can differentiate free from loculated pleural fluid, determine echogenicity of the fluid, demonstrate pleural thickening, and guide thoracentesis or chest drain placement. 1, 2
- Ultrasound is readily available, involves no radiation, requires no sedation, and is sufficient in the majority of cases. 3
- Fibrinous septations are better visualized using ultrasound than CT scans. 3
Step 3: CT Scanning (For Complicated Cases)
- Reserve contrast-enhanced CT chest for complicated cases, including failure to aspirate pleural fluid, failing medical management, or when diagnosis is uncertain. 1, 2, 4
- CT should be acquired 60 seconds after IV contrast bolus to optimize visualization of the pleura. 2
- Key CT findings include:
- The "split pleura" sign (enhancing pleural tissue visible on both visceral and parietal pleural surfaces) - highly suggestive of empyema 2, 5
- Pleural enhancement (sensitivity 84%, specificity 83%) 2
- Pleural thickening (sensitivity 68%, specificity 87%) 2
- Loculation (sensitivity 52%, specificity 89%) 2
- Extrapleural fat proliferation and increased attenuation 2
- CT helps delineate loculated pleural fluid, detect airway or parenchymal lung abnormalities (endobronchial obstruction, lung abscess), and assess mediastinal pathology. 3, 1
Diagnostic Thoracentesis and Pleural Fluid Analysis
Perform diagnostic thoracentesis in all patients with suspected pleural infection. 1
Biochemical Analysis
- pH: <7.2 indicates complicated parapneumonic effusion or empyema 1
- Glucose: <2.2 mmol/l suggests complicated effusion 1
- LDH: >1000 IU/l indicates complicated effusion 1
Microbiological Analysis
- Gram stain and aerobic/anaerobic cultures are essential, as anaerobes may be present in up to 76% of empyema cases, particularly following possible aspiration pneumonia. 1
- Frank pus on aspiration confirms empyema diagnosis. 1
Classification Based on Pleural Fluid
- Simple parapneumonic effusion: Clear fluid, pH >7.2, LDH <1000 IU/l, glucose >2.2 mmol/l, negative Gram stain/culture 1
- Complicated parapneumonic effusion: Clear or cloudy fluid, pH <7.2, LDH >1000 IU/l, glucose <2.2 mmol/l, may be positive Gram stain/culture 1
- Empyema: Frank pus, may be positive Gram stain/culture 1
Blood Cultures
Obtain blood cultures (including anaerobic bottle) in all patients with suspected empyema. 1, 2
- Blood cultures are positive in 10-22% of empyema cases. 3
Critical Pitfalls to Avoid
- Do not rely on ultrasound or CT to predict which patients will fail drainage and require surgery - neither technique reliably identifies the stage of pleural infection or predicts surgical need. 6
- Parapneumonic effusions <2.5 cm in anteroposterior dimension can often be managed without thoracentesis. 2
- Do not confuse empyema with lung abscess - use contrast-enhanced CT to distinguish between them, as treatment differs dramatically (empyema requires drainage; lung abscess should not be drained initially). 4