Pleural fluid analysis

A procedure called thoracentesis is used to get a sample of pleural fluid. Your health care provider sends the fluid to a lab where a technician tests the sample to look for:

• Cancerous (malignant) cells
• Other types of cells (for example blood cells)
• Levels of glucose, protein and other chemicals
• Bacteria, fungi, mycobacteria, viruses, and other germs that can cause infections
• Inflammation

No special preparation is needed before the test. An ultrasound, 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.

Tell your provider if you take medicines to thin the blood.

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.

During the test, tell your provider if you have sharp chest pain or shortness of breath.

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 to determine the cause of a pleural effusion. It is also done to relieve the shortness of breath that a large pleural effusion can cause.

Normally the pleural space contains less than 20 milliliters (ml) or 4 teaspoons (tsp) of clear, yellowish (serous) fluid.

Abnormal results may indicate possible causes of pleural effusion, such as:

• Cancer
• Cirrhosis
• Heart failure
• Infection
• Severe malnutrition
• Very low protein (albumin) in the blood
• Trauma
• Abnormal connections between the pleural space and other organs (for example, the esophagus)

If your provider suspects an infection, a culture of the fluid is done to check for bacteria and other microbes.

The test may also be performed for hemothorax. This is a collection of blood in the pleura.

Risks of thoracentesis are:

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

Serious complications are uncommon.

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.

McCool FD. Diseases of the diaphragm, chest wall, pleura, and mediastinum. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 86.

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: Jacob Berman, MD, MPH, Associate Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.