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

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

A sample called a smear from a skin lesion (sore) 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.

This test is often performed in a health care provider's office.

A scraping or biopsy of the skin sore 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.

The test is performed to determine if an infection is present in a skin sore, and if so, what bacteria is causing it.

The test is normal if no bacteria are identified.

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.

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

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.