Thoracentesis is a procedure in which a large needle is inserted into the pleural space so that excess pleural fluid can be removed. This is done for two reasons. One reason for this test is so that pain, pressure, and shortness of breath can be relieved with the removal of this excess fluid. Another use for the test is to help determine what is causing a buildup up pleural fluid. Because a large needle is being inserted into the pleural space, one of the risks associated with this procedure is a partially collapsed lung following the procedure. The medical term for this problem is pneumothorax.