A Gram stain of a skin lesion is a laboratory test that uses special stains to detect and identify bacteria in a sample from a skin sore. The Gram stain method is one of the most commonly used techniques to quickly diagnose bacterial infections.
Your health care provider will remove a sample of tissue from the skin sore. This procedure is called a skin lesion biopsy. Before the biopsy, your provider will numb the area of skin so you don't feel anything.
The sample is sent to a laboratory. There, it is applied in a very thin layer to a glass slide. A series of different colored stains is applied to the sample. The stained slide is examined under a microscope to check for bacteria. The color, size, shape, and organization of the cells help identify the germ causing the infection.
No preparation is needed for the laboratory test. Tell your provider if you have a history of bleeding problems because you may bleed slightly during the biopsy.
There will be a sting when the anesthetic is given. You should only feel pressure or discomfort similar to a pinprick during the biopsy.
Your provider may order this test if you have signs of an infected skin sore. The test is done to determine which bacteria caused the infection.
The test is normal if no bacteria are found.
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. Other tests may be done to help diagnose the problem.
An abnormal result means bacteria have been found in the skin lesion. Further tests are needed to confirm the results. This allows your provider to prescribe the appropriate antibiotic or other treatment.
Risks of a skin biopsy may include:
You will bleed slightly during the procedure.
A skin or mucosal culture may be done along with this test. Other studies are often done on a skin sample to determine if cancer is present.
Viral skin lesions, such as herpes simplex, are examined by other tests or a viral culture.
Reviewed By: Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.