Parapneumonic pleural effusions are divided into 3 groups or stages based on pathogenesis: uncomplicated parapneumonic effusion, complicated parapneumonic effusion, and thoracic empyema. Uncomplicated parapneumonic effusion is an exudative predominantly neutrophilic effusion that occurs as the lung interstitial fluid increases during pneumonia. These effusions are resolved with appropriate antibiotic treatment of pneumonia. Complicated parapneumonic effusion is a bacterial invasion of the pleural space that leads to an increased number of neutrophils, pleural fluid acidosis, and elevated lactic dehydrogenase concentration. These effusions often are sterile because bacteria are usually cleared rapidly from the pleural space. Thoracic empyema is characterized by either aspiration of pus on thoracentesis or the presence of bacterial organisms on Gram stain. A positive culture is not required for diagnosis. If the diagnostic thoracentesis yields thick pus, the patient has an empyema and institute tube thoracostomy immediately.