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Diagnostic Thoracentesis

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Diagnostic thoracentesis, or sampling of a pleural effusion using a needle through the chest wall, is a simple procedure done to look for the cause of a pleural effusion. Thoracentesis may also be used as a treatment to help relieve symptoms of an effusion.

Why is diagnostic thoracentesis done?

A pleural effusion is an abnormal collection of fluid in the pleural space surrounding the lungs. Diagnostic thoracentesis, or aspiration of a pleural effusion, is done to look for a cause for the effusion. The sample of fluid that is drained from the effusion can be analysed for the presence of infectious agents such as bacteria, or for special cell types that may suggest malignancy, as well as various other factors which may provide clues to the cause. (See this article for more information about causes of pleural effusions.) By draining some of the fluid from the effusion, thoracentesis may also relieve the symptoms caused by the effusion. These commonly include shortness of breath, chest pain, or dry cough.

What should I expect during the procedure?

Diagnostic thoracentesis is a simple procedure which can be done at a patient’s bedside. It does not require a general anaesthetic. Before thoracentesis is performed, a chest x-ray will usually be ordered to confirm the presence of a pleural effusion and to establish the precise location. Ultrasound may also be used during the procedure to guide needle insertion.

  • Patients are usually asked to sit upright during the procedure. It is important to remain still so that the needle is inserted into the correct place.
  • Antibacterial solution will be used to clean the skin around the needle insertion site. This is usually between the ribs at the back of the chest.
  • Local anaesthetic is injected into the back to help reduce discomfort.
  • A larger needle or catheter is then inserted in the same spot, passing deeper into the chest wall and into the pleural space. The pleural fluid draining through the needle will be collected in a bottle to be sent for analysis.
  • The needle or catheter will be removed, and a sterile dressing applied over the insertion site to help prevent infection.

If the patient develops a cough or chest pain at any time during the procedure, it should be stopped immediately. After the procedure, another chest x-ray may be needed to check for the presence of a pneumothorax (see ‘complications’ below).

What are the risks?

Risks of thoracentesis include:

  • Pneumothorax: this complication occurs in approximately one in ten cases. Many are very mild and require no treatment; some may require placement of a tube thoracostomy to drain the air.
  • Pain at the puncture site;
  • Bleeding;
  • Infection of the chest wall or pleural space (empyema);
  • Puncture of the spleen or liver;
  • Tumour seeding along the needle tract;
  • Re-expansion pulmonary oedema: this is a very rare complication occurring with drainage of very large volumes of fluid. Occasionally the lungs might react badly to the rapid re-expansion after thoracentesis, and the air spaces may fill with fluid. Re-expansion pulmonary oedema may be fatal.

Patients who have a bleeding disorder, or who are taking anticoagulant medications such as warfarin, may be at increased risk of bleeding during the procedure. Always tell your health provider if this applies to you. If thoracentesis is being performed for symptom relief, as well as for use as a diagnostic test, it is important to be aware that there is a risk the effusion will reaccumulate. This is particularly common in pleural effusions associated with malignancy. Repeat thoracentesis or placement of a chest tube (tube thoracostomy) may then be necessary.


  1. Hanley ME, Welsh CH. Current Diagnosis & Treatment in Pulmonary Medicine. McGraw-Hill, 2006.
  2. Rubins, J. ‘Pleural Effusion’ [online], 2005. Available at URL: (last accessed 6/9/06)
  3. Sahn, SA. ‘Diagnostic thoracentesis’ [online], UpToDate, 2005. Available at URL:
  4. Stone CK, Humphries RL. Current Emergency Diagnosis and Treatment. McGraw-Hill, 2006.

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Posted On: 8 September, 2006
Modified On: 27 January, 2014
Reviewed On: 5 April, 2008


Created by: myVMC