Pleural fluid Gram stain

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The pleural fluid Gram stain is a test to diagnose bacterial infections in the lungs.

Alternative Names

Gram stain of pleural fluid

How the Test is Performed

Normally the lungs fill a person's chest with air. If fluid builds up in the space outside the lungs but inside the chest, it can cause many problems. Removing the fluid can relieve a person's breathing problems and help explain how the fluid built up there.

A sample of the fluid can be removed for testing. This process is called thoracentesis. One test that can be done on the pleural fluid involves placing the fluid onto a microscope slide and mixing it with a violet stain (called a Gram stain). A laboratory specialist uses a microscope to look for bacteria on the slide.

If bacteria are present, the color, number, and structure of the cells are used to identify the type of bacteria. This test will be done if there is concern that a person has an infection involving the lung or the space outside the lung but inside the chest (pleural space).

Why the Test is Performed

The test is performed when the health care provider suspects an infection of the pleural space, or when a chest x-ray reveals an abnormal collection of pleural fluid.

Normal Results

Normally, no bacteria are seen in the pleural fluid.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

You may have a bacterial infection of the lining of the lungs (pleura).


Broaddus VC, Light RW. Pleural effusion. In: Mason RJ, Broaddus VC, Martin TR, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 5th ed. Philadelphia, PA: Elsevier Saunders; 2010:chap 73.

McCool FD. Diseases of the diaphragm, chest wall, pleura, and mediastinum. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 99.

Version Info

  • Last reviewed on 12/2/2014
  • Andrew Schriber, MD, FCCP, specialist in Pulmonary, Critical Care, and Sleep Medicine, Virtua Memorial Hospital, Mount Holly, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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