Thoracentesis is a medical procedure to remove fluid from the pleural cavity. It is used both for therapeutic purposes and for diagnostic purposes. It consists of inserting a large hollow needle through the wall of the thorax, usually under local anesthetic.
The procedure has been in use since the mid 19th century, but in modern medicine, ultrasound is used to guide the insertion of the needle.
The most common use of the procedure is when fluid fills the pleural cavity and there is no obvious underlying cause. In such cases, analysis of the fluid almost always provides a diagnostic clue to the patient's condition.