Gram stain of skin lesion

Alternative names
Skin lesion Gram stain; Skin biopsy Gram stain; Gram stain of skin biopsy


Gram stain is a method of staining microorganisms (bacteria) using a special series of stains. In this test, a specimen from a skin lesion is examined under the microscope.

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

How the test is performed

A sample called a smear from a skin lesion scraping or skin lesion biopsy is applied in a very thin layer to a microscope slide. A series of stains called a Gram stain is applied to the specimen. 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 identify the infecting organism.

How to prepare for the test
This test is often performed in a health care provider’s office.

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
A scraping or biopsy of the skin lesion will be taken. The skin area will be cleansed to avoid contamination with bacteria on the surface of the skin. A local anesthetic may be injected into the skin if a biopsy is taken. A pin-prick sensation may be felt as the anesthetic is injected. There may be a sensation of pressure at the site of the scraping or biopsy.

Why the test is performed
The test is performed to determine if infection is present in a skin lesion, and if so, the causative bacteria.

Normal Values
The test is normal if no bacteria are identified.

What abnormal results mean
The test is abnormal if bacteria are found in the skin lesion. The bacteria can sometimes be tentatively identified by the Gram stain. Culture is necessary to confirm the results.

What the risks are
The risks are minimal and may include bleeding at the lesion or infection.

Special considerations
A culture of the skin lesion may be performed in conjunction with the Gram stain (see skin or mucosal biopsy culture). Also, pathology studies are often done on a skin biopsy.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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