A sample of fluid from the pleural space is needed. The sample is taken using a procedure called thoracentesis.

The procedure 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.
• A small area of skin on your back is cleaned. Numbing medicine (local anesthetic) is injected in this area.
• The doctor inserts a needle through the skin and muscles of the chest wall into the pleural space.
• Fluid is collected.
• The needle is removed. A bandage is placed on the skin.

The fluid sample is sent to a laboratory. There, it is examined under the microscope to determine what the cells look like and whether they are abnormal.

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

Do not cough, breathe deeply, or move during the test to avoid injury to the lung.

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

Tell your health care provider if you feel short of breath or have chest pain.

A cytology exam is used to look for cancer and precancerous cells. It may also be done for other conditions, such as identifying systemic lupus erythematosus cells. Occasionally, microorganisms can be seen in cytology too, but in general, they are best identified by culture.

Your doctor may order this test if you have signs of fluid buildup in the pleural space. This condition is called pleural effusion. The test may also be done if you have signs of lung cancer.

Normal cells are seen.

In an abnormal result, there are cancerous (malignant) cells. This may mean there is a cancerous tumor. This test most often detects:

• Breast cancer
• Lymphoma (cancer of the lymph system)
• Lung cancer
• Ovarian cancer
• Stomach cancer

Risks are related to thoracentesis and may include:

• Bleeding
• Infection
• Collapse of the lung (pneumothorax)
• Difficulty breathing

Pleural fluid cytology; Lung cancer - pleural fluid

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.

Cibas ES. Pleural, pericardial, and peritoneal fluids. In: Cibas ES, Ducatman BS, eds. Cytology. 5th ed. Philadelphia, PA: Elsevier; 2021:chap 4.

Chernecky CC, Berger BJ. Thoracentesis - diagnostic. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:1052-1135.

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.