Thoracentesis is a procedure for withdrawing fluid from the lung by a technique called needle aspiration. A sterile needle is inserted into the lung at an area visualized by x-ray examination or other imaging technique and fluid is withdrawn, either to help relieve pressure from the build up of fluid or to obtain a specimen for diagnostic testing. When used for diagnostic purposes, thoracentesis may be necessary to obtain information that cannot be gained any other way.

Some of these situations include the following:

• When the reason for fluid in the lungs cannot be determined, or is inconclusive, by  other tests. For example, thoracentesis might be used when an infection is not responding to treatment and the type of microorganism cannot be identified by other means.

• When a malignancy is suspected, the thoracentesis may be used to obtain a biopsy of lung or pleural tissue to determine the type of cell abnormality and the most effective treatment

• When bleeding or other abnormal collection of fluid is suspected and cannot be identified by other diagnostic tests.

Thoracentesis is also used to relieve pressure in the chest when fluid build-up is of sufficient volume to restrict breathing, or when it is necessary to inject medications directly into a diseased area of the lung.