A thoracentesis involves taking a sample of fluid from the area surrounding the lungs. The lungs are covered by tissue called the pleura. The inside of the chest is also lined with the pleura. The space between these two areas is called the pleural cavity. Normally, this space contains only a thin layer of fluid. However, some disorders, such as pneumonia, tuberculosis, certain cancers, and heart and kidney failure, can cause excess fluid production (pleural effusion).

Imaging tests are usually done to see how much fluid is around the lungs. This may include a chest x-ray, a chest ultrasound, or a chest CT scan ("computed tomography") to see where the fluid is and how much is present. In some cases, the procedure may be done under ultrasound guidance.

In thoracentesis, a thin needle or tube is inserted into the pleural cavity to remove fluid. The needle or tube is inserted into the chest through the skin between the ribs. This procedure may be done to withdraw fluid for testing or treatment. Once the procedure is complete, the needle or tube is removed. The tube may remain in place for a longer period of time if more fluid needs to be removed.