Skin culture

Definition

A skin culture removes a small amount of skin, pus, or fluid from an infected wound. The sample is then grown in the lab to identify the organisms responsible for the infection. Sensitivity testing may be done on the organisms to help determine an effective treatment method.

Purpose

A skin culture is performed to help a doctor determine the cause of a skin infection. It is generally done for skin infections that have been present for a significant period of time or that have not responded to treatment. The culture determines if there is, in fact, an infection present, which can help rule out some other causes of the symptoms. In many cases sensitivity testing is also performed on the sample. Sensitivity testing is done to determine which treatment option is going to be most effective at killing the organisms causing the infection.

Precautions

If the patient is taking, or has recently been taking, antibiotics he or she should tell the doctor. Antibiotics can interfere with the ability of the infectious organism to grow on the culture. The patient should also inform the doctor if he or she has any allergies , as some people are allergic to certain types of local or topical anesthesia. The doctor will take precautions to ensure that the infection is not spread by taking the skin sample.

Description

Skin infections occur when a virus, bacterium, or fungus, that is not native to the skin begins to grow in or on the skin. It can be an organism from outside the body, or even an organism that occurs naturally in another part of the body but does not normally occur on or in the skin. Skin infections can occur on unbroken skin, but they are especially likely to occur on skin that has been broken by an injury or abrasive accident. Although most skin infections are not serious they can be very dangerous if left untreated.

To perform a skin culture a sample of the material from the area believed to be infected must be collected. There are a number of ways this may be collected. It may be swabbed or wiped with a sterile swab to collect any of the pus or excretion given off by the infected area. If the area believed to be infected is covered with a scab sample pus or liquid may be taken using a needle.

QUESTIONS TO ASK YOUR DOCTOR

• What type of organism is suspected?
• How can I help ensure my infection does not spread to other areas of my body?
• If the results come back negative what is the next step?

In some cases a sample of the skin itself, not just the pus or excretion, must be collected. In this case a skin biopsy is performed. There are three main types of biopsy. The first is a called a shave biopsy. In this case a very thin layer of the skin is shaved off and collected. The second type of biopsy is a punch biopsy. Punch biopsies are used when the doctor wants to test all layers of the skin, or the infection of the skin is not on the surface. A small instrument, usually about the size of a pencil eraser, is used to take a small, circular sample of the skin. The third type of biopsy is an excisional biopsy. In this case a section of skin is actually cut out to send for testing. Then the new wound site is either closed with stitches, or in some cases, a skin graft is used to cover the area.

After the sample has been collected it is sent to the laboratory. There a small amount of the sample is placed onto a culture medium and allowed to grow for anywhere from one to two days to a few weeks. The laboratory technician then uses a microscope and a variety of testing procedures to identify any colonies that have formed.

In some cases sensitivity testing is also performed. In that case the laboratory technician exposes colonies that have grown on the sample to a variety of antibacterial, antiviral, or antifungal agents until one is identified that successfully kills the organisms present. This can then be used as an effective treatment for the infection.

Preparation

There is no special patient preparation required before a skin culture. Patients can eat and drink normally, and participate in normal daily activities before the culture is performed.

Aftercare

Most skin culture require little or no aftercare. There may be temporary discomfort at the site if the site is tender from the infection. If a biopsy is needed to collect the sample additional aftercare may be required. If a punch biopsy is performed stitches may or may not be required. If not, the area will be bandaged and allowed to heal on its own, but will require regular cleaning and good would hygiene to promote healing. If the punch biopsy encompasses a large area, stitches may be used to close the wound. In this case the stitches must be kept clean, and the bandage may need to be changed regularly. Most stitches are removed in three to 14 days. If the biopsy is excisional it will also be closed with stitches and will require care to stay clean and dry. The doctor may prescribe antibiotic ointment to be rubbed onto the wound regularly to help prevent additional infection from occurring.

Complications

In most cases of skin culture there are no complications. There is always a small chance of infection, bruising , or bleeding, anytime an injection is given. Therefore if local anesthesia is given there is a small chance of these complications. If a section of skin needs to be removed there is a chance of new infection, slow healing, scarring, and excessive bleeding at the removal site. There is also a very small chance that the infection could spread, especially if a biopsy is required.

Results

A normal skin culture result shows that no other organisms have grown on the culture medium other than those normally present on or in the skin. A normal result indicates that no infection is present. Abnormal results occur when there is a much larger than normal amount of organism growth on the culture medium, or one type of organism grows much more readily than any other organisms present. Abnormal results indicate a skin infection.

Once it has been determined that a large colony or organisms grow in the culture medium, sensitivity testing often occurs. The results of sensitivity testing indicate the effectiveness of various antibiotics, antiviral, or antifungal agents that were applied to the growing organisms. Positive results from a sensitivity test indicate that the organisms exposed to the agent were successfully killed. Negative results indicate that the agent had little or no effect on the health and reproduction of the organisms present.

Caregiver concerns

The doctor who determined the need for the skin culture often performs the sample or biopsy him or herself. A nurse may perform sample if a swab is performed. The doctor or nurse then labels the sample and sends it to the laboratory, where a laboratory technician performs the culture itself. The laboratory technician also performs the sensitivity testing if required. The results are then sent back to the doctor who ordered the skin culture who determines and prescribes treatment as necessary.

KEY TERMS

biopsy —the diagnostic removal of a sample of tissue from a living body

Resources

books

Frankel, David H., ed. Field Guide to Clinical Dermatology, 2nd Ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.

Jablonski, Nina G. Skin: A Natural History. Berkeley: University of California Press, 2006.

Noble, W.C., ed. The Skin Microflora and Microbial Skin Disease. Cambridge, England: Cambridge University Press, 2004.

periodicals

MacNeil, Jane Salodof. “Shave Biopsy May Impair Accuracy.” Family Practice News 37.10 (May 15, 2007):24–25.

Snyder, Robert J. “Clinical Evaluation of Wound Swabbing Versus Tissue Biopsy to Diagnose Infection.” Podiatry Management 26.7 (Sept 2007): 217–224.

organizations

American Skin Association, 346 Park Avenue South, 4th Floor, New York, NY, 10010, 800-499-7546, http://www.americanskin.org/.

Robert Bockstiegel