Diagnostic Workup for Empyema Necessitans

Initial Imaging Approach

The American College of Radiology recommends chest radiography as the initial imaging study to detect pleural effusion and assess for pulmonary infiltrates, with a sensitivity of 83.9% for detecting pleural effusions when using posteroanterior and lateral views 3
PA and lateral radiographs have significantly greater sensitivity (83.9%) compared to single-view AP radiographs (67.3%) for detecting pleural effusions, according to the American College of Radiology 3
Most missed parapneumonic effusions on radiographs occur in patients with coexistent lower lobe consolidation, as reported by the American College of Radiology 3
Chest ultrasound must be performed to confirm the presence of pleural fluid collection and guide further management, with the ability to differentiate free from loculated pleural fluid, determine echogenicity of the fluid, demonstrate pleural thickening, and guide thoracentesis or drain placement, as recommended by the British Thoracic Society 5

The American College of Radiology recommends CT chest with IV contrast in cases of suspected empyema necessitans, with key CT findings including pleural enhancement (sensitivity 84%, specificity 83%), pleural thickening (sensitivity 68%, specificity 87%), loculation (sensitivity 52%, specificity 89%), extrapleural fat proliferation (sensitivity 53%, specificity 91%), and increased attenuation of extrapleural fat (sensitivity 39%, specificity 97%) 1, 8
Technical considerations for CT chest include acquiring the scan 60 seconds after IV contrast bolus to optimize visualization of the pleura, as recommended by the American College of Radiology 1, 2

Blood cultures, including anaerobic bottle, should be obtained, and pleural fluid analysis should include Gram stain and culture, biochemical tests (pH, glucose, LDH), and acid-fast bacilli stain and culture, as recommended by the British Thoracic Society 6, 9
Additional blood tests, such as complete blood count, C-reactive protein, and serum albumin, can be useful in the diagnosis and management of empyema necessitans, as reported by the British Thoracic Society 6

The "split pleura" sign on contrast-enhanced CT, which is indicative of empyema, is characterized by enhancing pleural tissue visible on both visceral and parietal pleural surfaces, as described by the British Thoracic Society 9
Parapneumonic effusions <2.5 cm in anteroposterior dimension can often be managed without thoracentesis, according to the American College of Radiology 3, 1