The doctor may use a cotton swab to collect a sample from an open skin rash or skin sore.
A sample of skin may need to be taken. This is called a skin biopsy. Before the skin sample is removed, you will likely receive a shot (injection) of numbing medicine to prevent pain.
A small sample of a fingernail or toenail may be taken. It may take up to three weeks to get results for this type of culture.
The sample is sent to a lab. There, it is placed in a special dish (culture). It is then watched to see if bacteria, virus, or fungus grow. It may take up to three weeks to get results for a nail culture. Further tests can be done to identify the specific germ that is causing your problem. This can help your doctor determine the best treatment.
How to Prepare for the Test
There is no preparation needed for this test.
If a skin or mucosal sample is needed, your provider will tell you how to prepare.
How the Test will Feel
If a skin sample is taken, you may feel a sting when the shot of numbing medicine is given.
For a nail sample, the doctor scrapes the affected area of the nail. There is usually no pain.
Why the Test is Performed
This test may be done to diagnose the cause of:
A bacteria or fungus infection of the skin, finger, or toenail
A skin rash or sore that appears to be infected
A skin ulcer that is not healing
A normal result means no disease-causing germs are seen in the culture.
Some germs normally live on the skin. These are not a sign of infection and are considered a normal finding.
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.
What Abnormal Results Mean
An abnormal result means bacteria, fungus, or virus is present. This may be a sign of infection.
Common skin infections caused by bacteria include:
Risks include slight bleeding or infection in the area where the skin sample was removed.
Croft AC, Woods GL. Specimen collection and handling for diagnosis of infectious diseases. In: McPherson RA, Pincus MR, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, Pa: Elsevier Saunders; 2011:chap 63.
High WA, Tomasini CF, Argenziano G, Zalaudek I. Basic principles of dermatology. In: Bolognia JL, Jorizzo JL, Schaffer JV, eds. Dermatology. 3rd ed. Philadelphia, Pa: Elsevier Saunders; 2012:chap 0.
Linda J. Vorvick, MD, Medical Director and Director of Didactic Curriculum, MEDEX Northwest Division of Physician Assistant Studies, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.