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Strep throat testing

Also known as: Throat culture
Formal name: Group A streptococcus, group A beta haemolytic streptococcus, streptococcus pyogenes

At a Glance

Why Get Tested?

To diagnose a streptococcus pyogenes infection of the throat (strep throat)

When to Get Tested?

If you have a sore throat and fever and your doctor thinks it may be due to a bacterial cause

Sample Required?

A swab brushed against your throat and tonsils

The Test Sample

What is being tested?

The test identifies Streptococcus pyogenes, known as Group A streptococcus, which are bacteria that infect the back of the throat and cause the common infection called strep throat.

How is the sample collected for testing?

The bacteria are tested for by throat culture. Your doctor may use a tongue depressor to hold down your tongue, and then s/he will insert a special swab into your mouth and brush it against your throat and tonsils. The swab will be sent to a laboratory, where culture is performed.

The Test

How is it used?

The majority of sore throats are caused by a virus and do not require active treatment with antibiotics. Some, caused by the streptococcus pyogenes bacterium, may be severe and are treatable with antibiotics. Your doctor may use a throat culture if s/he thinks you might have strep throat to help make the correct diagnosis. This allows your doctor to prescribe the proper antibiotics for treatment. A throat culture may take several days to get results.

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When is it requested?

Your doctor may request this test if you have a sore throat and a fever that might be due to a bacterial infection. Other symptoms include:

  • tonsils that may appear red with white or yellow spots at the back of the throat
  • a swollen, tender neck
  • skin rash
  • weakness
  • loss of appetite

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What does the test result mean?

A positive throat culture indicates the presence of group A streptococci, the bacteria that cause strep throat.

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Is there anything else I should know?

Strep throat may be contagious and spreads through person-to-person contact. Symptoms usually occur within two to seven days after exposure. If untreated, this throat infection can create serious lifelong problems in a small minority of patients.

Strep throat is most common in 5- to 10-year-olds. Up to 20% of school children may be 'carriers' - people who have the bacteria but who have no symptoms. Carriers can still spread the infection to others.

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Common Questions

1.  How long does treatment for strep throat usually last?

Ten to 14 days

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2.  How long should I stay away from other people if I have a positive test result?

Twenty-four hours after starting medication

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3.  When can my child go back to school?

Usually after one full day of therapy and absence of significant fever

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4.  If one child in my family has strep throat, is everyone going to get sick?

Other family members, including adults, can contract the bacteria.

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5.  What is an ASO test and how is it used to detect a strep infection?

Antistreptolysin O (ASO) titre is a blood test used to help diagnose a current or past infection with Group A strep (Streptococcus pyogenes). It detects antibodies to streptolysin O, one of the many strep antigens. This test is rarely used now compared to thirty years ago. For an acute strep throat infection, this test is not performed; the throat culture is used.

However, if a doctor is trying to find out if someone had a recent strep infection that may not have been diagnosed, this test could be helpful. In addition, it may be used to help diagnose rheumatic fever, which occurs weeks after a strep throat infection when the throat culture would no longer be positive.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.