Pleural fluid smear

A procedure called thoracentesis is used to get a sample of pleural fluid. Your health care provider examines a sample of pleural fluid under the microscope. If bacteria, mycobacteria, or fungi are found, other methods may be used to further identify those organisms.

No special preparation is needed before the test. An ultrasound, chest CT scan, or chest x-ray will be performed before and after the test.

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

For thoracentesis, you sit on the edge of a chair or bed with your head and arms resting on a table. Your provider cleans your skin around the insertion site. Numbing medicine (anesthetic) is injected into your skin.

A needle is placed through the skin and muscles of your chest wall into the space around the lungs, called the pleural space. As fluid drains into a collection bottle, you may cough a bit. This is because your lung re-expands to fill the space where fluid had been. This sensation may last for a few hours after the test.

Ultrasound is often used to decide where to insert the needle and to get a better view of the fluid in your chest.

The test is performed if you have a pleural effusion and its cause is not known, especially if your provider suspects an infection or cancer.

Normally, no bacteria, fungi, or cancer cells are present in the pleural fluid.

Normal value ranges may vary slightly among different laboratories. Talk to your provider about the meaning of your specific test results.

Positive results may indicate that infection, or cancer cells, are present. Other tests can help identify the specific type of infection or cancer. Sometimes, the test may show abnormalities (such as special types of cells) from conditions such as systemic lupus erythematosus.

Risks of thoracentesis are:

• Collapsed lung (pneumothorax)
• Excessive loss of blood
• Fluid re-accumulation
• Infection
• Pulmonary edema
• Respiratory distress
• Cough that doesn't go away

Broaddus VC, Light RW. Pleural effusion. In: Broaddus VC, Ernst JD, King TE, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 7th ed. Philadelphia, PA: Elsevier; 2022:chap 108.

Ruhl TS, Good JL. Thoracentesis. In: Fowler GC, ed. 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.