Chest x-rays reveal pleural effusions that are free flowing in early disease but frequently loculated in late cases. Lateral decubitus films or chest fluoroscopy can often show the presence of fluid, but when effusions are loculated, ultrasonography may be necessary to distinguish fluid from pleural fibrosis. Unless surgery or thoracentesis has been performed, air-fluid levels in the pleural space suggest a bronchopleural fistula. In the differential diagnosis of empyema, it is important to consider the many causes of noninfected pleural effusions. Most important is the distinction between sterile parapneumonic effusions and true empyemas.
Thoracentesis is mandatory for the diagnosis of empyema.
Gross purulence is diagnostic for empyema, but the absence of frank pus does not rule out infection. Pleural fluid acidosis is characteristic of empyemas and has been reported in loculated sterile effusions, but pH values less than 7.3 have not been found in benign effusions that resolve spontaneously. Pleural fluid alkalosis can occur in empyema if the infection is caused by a urea-splitting organism such as Proteus . Gram's stains of the pleural fluid will often reveal the causative organisms. Stains and cultures for mycobacteria and fungi are important in selected cases.