To definitively determine the cause of a pleural effusion or to relieve the symptoms associated with a pleural effusion, a thoracentesis (or pleural tap) may be done. This procedure can be performed safely on patients when sufficient fluid is present in the pleural space. The skin on the back is numbed with a topical anesthetic and a needle is inserted through the tissue in the back (between the ribs) until the fluid in the pleural space is reached. During a diagnostic thoracentesis a small amount of fluid is removed for tests while a therapeutic thoracentesis removes a larger amount fluid for symptom relief. The fluid is then tested and categorized into either a transudate or exudate (see pleural effusion).