Pleural fluid analysis is a test that examines a sample of fluid that has collected in the pleural space. This is the space between the lining of the outside of the lungs (pleura) and the wall of the chest. When fluid collects in the pleural space, the condition is called pleural effusion.
How the Test is Performed
A procedure called thoracentesis is used to get a sample of pleural fluid. The health care provider examines 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
How to Prepare for the Test
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.
How the Test will Feel
For thoracentesis, you sit on the edge of a chair or bed with your head and arms resting on a table. The provider cleans the skin around the insertion site. Numbing medicine (anesthetic) is injected into the skin.
A needle is placed through the skin and muscles of the chest wall into 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 lasts 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 the needle is inserted and to get a better view of the fluid in your chest.
Why the Test is Performed
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 cavity contains less than 20 milliliters (4 teaspoons) of clear, yellowish (serous) fluid.
What Abnormal Results Mean
Abnormal results may indicate possible causes of pleural effusion, such as:
- Heart failure
- Severe malnutrition
- Abnormal connections between the pleural space and other organs (for example, the esophagus)
If the 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.
Blok BK. Thoracentesis. In: Roberts JR, Custalow CB, Thomsen TW, eds. Roberts & Hedges' Clinical Procedures in Emergency Medicine and Acute Care. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 9.
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.
Review Date 12/6/2021
Updated by: Denis Hadjiliadis, MD, MHS, Paul F. Harron, Jr. Associate Professor of Medicine, Pulmonary, Allergy, and Critical Care, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.