Gram stain of pleural fluid

Alternative names
Pleural fluid gram stain

The pleural fluid gram stain is a method of staining microorganisms using a violet stain. In this test, a small amount of pleural fluid - found in the space surrounding the lungs - is examined under the microscope. The gram stain method can be applied to almost any clinical specimen, and it is one of the best techniques for the rapid diagnosis of bacterial infections.

How the test is performed
A sample of pleural fluid obtained by thoracentesis (needle aspiration) is applied in a very thin layer to a microscope slide. The specimen is stained with a violet stain, known as a gram stain, and examined under the microscope for the presence of bacteria. The color, number, and structure of the cells help to identify the organism.

How to prepare for the test
It is important not to cough, breathe deeply, or move when the fluid sample is being taken. There is no other special preparation for the test.

Infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following topics:

  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel

The specimen is obtained by thoracentesis. An area on the chest is cleansed with antibacterial soap and numbed with local anesthetic. A needle is placed between the ribs, and a sample of fluid is withdrawn from the chest.

You may feel a stinging sensation when the anesthetic in injected. You may feel some pressure and slight localized pain when the thoracentesis needle enters the pleural space. A chest x-ray is usually done following the test to be sure the lung tissue was not affected by the test.

Why the test is performed
The test is performed when infection of the pleural space is suspected or when an abnormal collection of pleural fluid is noticed by chest x-ray.

Normal Values

Normally, no organisms are present in the pleural fluid.

What abnormal results mean
A bacterial infection of the pleura (lining of the lungs) may be present.

What the risks are
There is a risk of internal bleeding into the lung and pneumothorax (collapsed lung). Serious complications are extremely rare.

Special considerations

The fluid smear cost and the cost of the procedure to obtain the specimen are charged separately. Other tests may be done on the sample of fluid obtained.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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