Test Overview

A skin or wound culture is a test to find germs (such as bacteria or a
fungus) that can cause an infection. A sample of skin, tissue, or fluid is added to a substance that promotes the growth of germs. If no germs grow, the
culture is negative. If germs that can cause
an infection grow, the culture is positive. The type of
germ may be identified with a microscope or chemical tests. Sometimes other tests are done to find the right medicine for treating the infection. This is called sensitivity testing.

Most bacteria can grow in oxygen. They are called aerobic bacteria and
usually are found in wounds close to the skin surface (superficial). Bacteria
that cannot grow in the presence of oxygen (anaerobic) usually are found in
deeper wounds and
abscesses. A wound culture can find out whether
bacteria are aerobic or anaerobic.

A fungal culture is done to
find out if an infection is caused by a fungus. A
viral culture can be done to find out whether an
infection is caused by a virus.

Some types of bacteria that
normally live on or in the body can cause an infection if they go to parts of
the body where they are not normally found. For example, E. coli bacteria are normally found in the colon and anus. But if
E. coli bacteria spread from the
anus to the
urethra, the bacteria may cause a
urinary tract infection (UTI).

Culture samples may
also be collected from the ear or eye, from open or closed sores, or from nails
and hair.

Why It Is Done

A skin or wound culture is done
to:

  • Find the cause of an infection in a sore, burn,
    surgical wound, or injury. An injury includes animal bites, human bites, marine
    stings or scrapes, cuts, and puncture wounds that are more likely to get
    infected.
  • Make decisions about the best treatment for an infection.
    This is called sensitivity testing.

How To Prepare

You do not need to do anything before
you have this test. If you are taking or have recently taken
antibiotics, tell your doctor.

Talk to
your doctor about any concerns you have regarding the need for the test, its
risks, how it will be done, or what the results will mean. To help you
understand the importance of this test, fill out the
medical test information form (What is a PDF document?).

How It Is Done

To collect a tissue or fluid sample from
a wound, a sterile swab is inserted into the wound. The health professional
collecting the sample may press around the wound and gently turn the swab to
collect as much tissue or fluid as possible. The swab is then placed into
either an aerobic or anaerobic culture tube or both, depending on the type of
organism suspected.

A needle may be used to collect fluid from a
wound that is covered (scabbed-over) or from an abscess. The fluid is then
placed in the culture tube.

Your doctor may need to remove a
sample of skin or tissue (biopsy) for testing. If collecting the
sample is likely to cause pain, you may be given a shot to numb the area (local anesthetic) first.

Once a sample is
collected, it is placed in a container with a substance (called growth medium
or culture medium) that helps bacteria, fungus, or viruses grow.

  • Bacteria usually need about 1 to 2 days to grow.
  • Fungi usually need several days to grow.
  • Viruses need to be placed in a container with living cells and can take weeks to grow.

Any
bacteria, fungi, or viruses that grow will be identified with a microscope, chemical
tests, or both. If sensitivity testing is done to help make decisions about
treatment, more time will be needed.

How It Feels

If you have a sample of fluid or tissue
collected from a wound, you may feel some pain when the sample is collected.
You may feel a short, sharp sting if you are given a shot of anesthetic to numb
the area where the culture sample will be taken.

Risks

There is a very slight risk of spreading some
infections if a biopsy is needed to collect the sample.

Results

A skin or wound culture is a test to find
and identify germs (such as bacteria, a
fungus, or a virus) that may be growing on the skin or
in a wound.

Some types of bacteria, fungi, and viruses grow
quickly in culture, and some grow slowly. Test results may take from one day to
several weeks, depending on the type of infection suspected.

Skin and wound cultures
Normal:

No large numbers of harmful germs are found
on the skin or in the wound. Normal culture results are negative.

Abnormal:

Harmful germs are found on the skin or in
the wound. Abnormal culture results are positive.

If test results are positive,
sensitivity testing may be done help make decisions
about treatment.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Taking or having recently taken antibiotics.
  • Getting bacteria that is normally found on the skin in the
    tissue or wound sample.

What To Think About

  • Types of bacteria that commonly cause infection
    in wounds are staph (Staphylococcus), strep (Streptococcus),
    and Clostridium perfringens, a bacterium found in soil
    and in stool (feces) that can cause
    gangrene. The most common type of fungus that causes
    infection in wounds is Candida albicans.
  • Looking at fluid (such as pus) from a wound under
    a microscope can sometimes help identify the type of bacteria or fungi causing
    the infection before culture results are ready.
  • A culture that does
    not grow any bacteria may not mean that you do not have an infection. Sometimes
    the amount of sample collected, the age of the wound (or skin problem), the
    type of culture done, and previous use of antibiotics can prevent the growth of
    bacteria in the culture.
  • Testing for a virus may be done to detect
    and identify a viral infection in the body that is causing symptoms. To learn more, see the topic
    Viral Tests.
  • Most types of fungi grow very
    slowly and may not show up in a culture for several weeks. Your doctor may
    recommend that you start treatment before your culture results come back if he
    or she thinks you could have a fungal infection.
  • Sensitivity
    testing helps your doctor choose the best medicine to treat specific types of
    bacteria, viruses, or fungus.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

Credits

ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Specialist Medical Reviewer Elizabeth T. Russo, MD – Internal Medicine

Current as ofMarch 3, 2017