An empyema is, by definition, pus in the pleural space. It occurs when the concentration of leukocytes becomes macroscopically evident as a thick and turbid fluid.
In more than 50% of cases, empyema is of parapneumonic origin. Other common causes include surgical procedures, traumas and esophageal perforation.
In all these cases, Gram stain and bacterial cultures are usually positive.
The most common Gram-positive bacteria isolated in culture are streptococci (usually S. pneumoniae). In rare instances, empyema has a noninfectious cause.
What are the lab findings diagnostic of an empyema?