Gram stain of pericardial fluid

Alternative names
Pericardial fluid Gram stain


This is a method of staining microorganisms (bacteria) using a special series of stains. In this test, a specimen of pericardial fluid (fluid in the sac surrounding the heart) is stained and then examined under the microscope.

The Gram stain method can be applied to almost any clinical specimen, and it is one of the most commonly used techniques for the rapid diagnosis of bacterial infections.

How the test is performed

The fluid must first be obtained from the sac surrounding the heart. In some people, a cardiac monitor may be placed prior to the test to monitor for heart disturbances. Patches called electrodes will be placed on the chest similar to an ECG. A chest X-ray or ultrasound may precede the test.

The skin of the chest will be cleansed with antibacterial soap. A trained physician, often a cardiologist, will obtain the sample. A small needle is inserted into the chest between the ribs into the pericardium, the thin sac that surrounds the heart, and a small amount of fluid is withdrawn. There will be a sensation of pressure and some pain as the needle is inserted into the chest.

An ECG and chest X-ray may also follow the procedure. Sometimes the pericardial fluid is obtained during open heart surgery.

A drop of the pericardial fluid is then applied in a very thin layer to a microscope slide - this is called a smear. The specimen is stained with a series of stains, together called a Gram stain. It is first stained with crystal violet stain, then iodine, then decolorized, then stained with safranin. The stained smear is then examined under the microscope for the presence of bacteria.

The color, size, and morphologic appearance (shape) of the cells help make it possible to identify the infecting organism.

How to prepare for the test
Food and fluid will probably be restricted for several hours before the test. A chest X-ray or ultrasound may precede the test to identify the area of fluid collection.

For infants and children:
The preparation you can provide for this test depends on your child’s age and experience. For specific 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
There will be some pressure and discomfort associated with the removal of the fluid from the sac surrounding the heart. Your doctor should be able to give you enough pain medicine so that the procedure does not hurt very much.

Why the test is performed
The test is performed when an infection of the heart is suspected or when a pericardial effusion is present and the cause is unknown.

Normal Values
The absence of organisms in the Gram stain is normal.

What abnormal results mean
An infection of the pericardium or heart may be present, if bacteria are seen. A specific organism may be identified by further testing, such as bacterial culture. Sometimes serology testing can help with the diagnosis.

What the risks are
There is a risk of puncturing the heart or the lung with this procedure. Introducing infection into the area is another rare, but potential risk.

Special considerations
Other tests may be performed on the pericardial fluid sample.

Johns Hopkins patient information

Last revised: December 7, 2012
by Mamikon Bozoyan, M.D.

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