These occurs 2° to pneumonia.
It is not necessary to have proven bacterial pneumonia; clinical Dx is enough.
Classification of parpneumonic effusions:
Risk of poor outcome/Pleural space anatomy/pH/Bacteriology (gram stain and culture)/Drainage indicated:
Very low / minimal free-flowing effusion (<10 mm on lateral decubitus) AND pH >7.2 AND negative or unkonwn bateriology. No drainage indicated
Low / small to moderate free-flowing effusion (>10 mm and <1/2 hemithorax) AND >7.2 or more AND negative bacteriology. No drainage.
Moderate / large free-flowing or loculated effusion >1/2 or more of hemithorax OR <7.2 OR positive bacteriology, glu <40 mg/dL.. Drainage indicated.
High / pus / drain.