The test is done in the following way:

• You sit on a bed or on the edge of a chair or bed. Your head and arms rest on a table.
• The skin around the procedure site is cleaned. A local numbing medicine (anesthetic) is injected into the skin.
• A needle is placed through the skin and muscles of the chest wall into the space around the lungs, called the pleural space. The health care provider will likely use ultrasound to find the best spot to insert the needle.
• You may be asked to hold your breath or breathe out during the procedure.
• You should not cough, breathe deeply, or move during the test to avoid injury to the lung.
• Fluid is drawn out with the needle.
• The needle is removed and the area is bandaged.
• The fluid may be sent to a laboratory for testing (pleural fluid analysis).

No special preparation is needed before the test. A chest x-ray or ultrasound will be done before and after the test.

You will feel a stinging sensation when the local anesthetic is injected. You may feel pain or pressure when the needle is inserted into the pleural space.

Tell your provider if you feel short of breath or have chest pain, during or after the procedure.

Normally, very little fluid is in the pleural space. A buildup of too much fluid between the layers of the pleura is called a pleural effusion.

The test is performed to determine the cause of the extra fluid and to relieve symptoms from the fluid buildup.

Normally the pleural cavity contains only a very small amount of fluid.

Testing the fluid will help your provider determine the cause of pleural effusion. Possible causes include:

• Metastatic cancer
• Liver failure
• Heart failure
• Low protein levels
• Kidney disease
• Trauma or post-surgery
• Asbestos-related pleural effusion
• Collagen vascular disease (class of diseases in which the body's immune system attacks its own tissues)
• Drug reactions
• Collection of blood in the pleural space (hemothorax)
• Lung cancer 
• Swelling and inflammation of the pancreas (pancreatitis)
• Pneumonia
• Blockage of an artery in the lungs (pulmonary embolism)
• Severely underactive thyroid gland

If your provider suspects that you have an infection, a culture of the fluid may be done to test for bacteria.

Risks may include any of the following:

• Bleeding
• Infection
• Collapsed lung (pneumothorax)
• Respiratory distress

A chest x-ray or ultrasound is commonly done after the procedure to detect possible complications.

Pleural fluid aspiration; Pleural tap

Blok BK. Thoracentesis. In: Roberts JR, Custalow CB, Thomsen TW, eds. Roberts and Hedges' Clinical Procedures in Emergency Medicine and Acute Care. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 9.

Ruhl TS, Good JL. Thoracentesis. In: Fowler GC, Choby BA, Iyengar D, et al, eds. Pfenninger and Fowler's Procedures for Primary Care. 4th ed. Philadelphia, PA: Elsevier; 2020:chap 218.

Updated by: Denis Hadjiliadis, MD, MHS, Paul F. Harron, Jr. Professor of Medicine, Pulmonary, Allergy, and Critical Care, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.