Strep Throat Test
Also known as:Throat Culture; Culture, Throat; Rapid Strep Test; Rapid Antigen Detection Test; RADT; Streptococcal Screen
Formal name:Group A streptococcus (GAS); Group A beta hemolytic streptococcus
Related tests:Influenza Tests; Blood Culture; ASO; Anti-DNase B
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Why Get Tested?
To determine if your sore throat is "strep throat" that is caused by group A streptococcus (GAS) bacteria
When to Get Tested?
When you have a sore throat that starts quickly and lasts more than a week and/or other symptoms such as a fever of 101° F or higher, or reddened throat and/or tonsils with white or yellow patches or streaks
A swab rubbed against the back of your throat and tonsils
Test Preparation Needed?
How is it used?
Strep tests are used to determine whether a person with a sore throat (pharyngitis) has strep throat, an infection of the throat and tonsils caused by the bacteria Streptococcus pyogenes, also called group A streptococcus (GAS).
Most sore throats are caused by a virus and will resolve without treatment within a few days, but some people with sore throats have strep throat. It is important to diagnose and treat strep infections promptly with antibiotics because they are very contagious and secondary complications can develop, especially in children.
A rapid strep test, also known as a rapid antigen detection test (RADT), can detect group A strep antigens. Results are available in 10-20 minutes. Recently, the U.S. Food and Drug Administration (FDA) approved molecular tests that can detect genetic material from group A strep bacteria in throat swab specimens in less than 8 minutes.
If the results of the rapid test are positive, further testing is not needed and treatment can be started right away. If the rapid strep test is negative, a throat culture should be performed on children or adolescents to confirm the results and avoid missing infections that could lead to serious complications, such as rheumatic fever. A throat culture is more sensitive than the rapid strep test, but it may take 24-48 hours for results. According to 2012 guidelines from the Infectious Diseases Society of America (IDSA), confirmatory testing on adults is not usually recommended since adults have lower rates of strep throat and far lower risk of complications than children.
When is it ordered?
A health practitioner will typically order this test when a person has a sore throat and other symptoms that suggest strep throat. There is a higher suspicion of strep when the affected person is a child and/or if the person has been in close contact with someone who has been diagnosed with strep throat. Individuals should see a healthcare provider and get tested when they have:
- Sore throat that starts quickly and lasts more than a week, or recurrent sore throats
- Fever of 101° F or higher
- Reddened (inflamed) throat and/or tonsils with white or yellow patches or streaks
- Tiny red spots at the back of the roof of the mouth
- Difficulty or severe pain when swallowing
- Headache, body aches
- Nausea or vomiting
- Tender and/or swollen throat or lymph nodes
- Hoarseness lasting more than two weeks
- Blood in saliva or mucus
- Excessive drooling in young children
- Symptoms of dehydration, such as excessive thirst, dry mouth, decreased urination
In accordance with the 2012 Infectious Diseases Society of America (IDSA) guidelines, testing is not recommended when there are also symptoms more closely associated with a viral infection, such as:
- Runny nose
- Mouth sores
Since strep is less common in children under the age of 3, IDSA advises against strep throat testing in this age group unless the child is considered at risk, such as when a family member has been diagnosed with strep throat. In general, the IDSA advises against the routine screening of asymptomatic people who have been in close contact with someone with a strep throat infection.
What does the test result mean?
A positive rapid strep test indicates the presence of group A streptococci, the bacteria that cause strep throat. A negative rapid test indicates that the affected person probably does not have strep throat, but a throat culture may be performed for confirmation, especially among children and adolescents.
If the throat culture is positive for group A streptococci, then the person tested does have strep throat. In rare cases, pharyngitis may be caused by group C or group G streptococci. These organisms will not cause a positive rapid test but can be detected by culture. If the culture is negative, then it is most likely that the sore throat is due to a viral infection that will resolve on its own.
Is there anything else I should know?
Strep throat spreads from person-to-person through contact with respiratory secretions that contain the streptococcal bacteria. During influenza season, the early symptoms of influenza, such as fever, chills, headache, sore throat, and muscle pain, may mimic strep throat. To differentiate between strep and influenza, a rapid strep test and a rapid influenza test may be done at the same time.
Most people with streptococcal pharyngitis would eventually recover without antibiotic treatment, but they will be contagious for a longer period of time and are at a greater risk of developing secondary complications.
Strep throat is most common in 5 to 15 year olds. Some school children may be "carriers," persons who have the bacteria but who have no symptoms. Carriers can still spread the infection to others.
Recent antibiotic therapy or gargling with some mouthwashes may affect the rapid strep test results.
One note: the 2012 IDSA guidelines are voluntary and do not represent universal consensus in the medical community on when and how to use the strep test. They reflect current knowledge and expert opinion and are intended to assist in the diagnosis and management of strep throat.
What is being tested?
The bacteria Streptococcus pyogenes, also known as group A beta-hemolytic streptococcus or group A streptococcus (GAS), causes "strep throat," the most common bacterial cause of inflammation and soreness of the back of the throat (pharyngitis). Strep throat tests identify the presence of these bacteria as the cause of a sore throat.
While most sore throats are caused by a virus and will resolve without treatment within a few days, some people with sore throats have strep throat. Strep throat is most common in children ages 5 to 15 years old. It is important that these bacterial strep infections be promptly identified and treated with antibiotics.
Strep throat is contagious and can spread when an infected person coughs or sneezes and other people come into contact with the droplets or mucus. Touching the face, eyes or mouth after touching something that has these droplets on it can spread the infection. The best way to avoid getting strep throat is to wash hands thoroughly and often and avoid sharing items like utensils or cups. A person who has a sore throat should wash their hands often and cover their mouth with a tissue or sleeve when coughing and sneezing.
If strep throat is not diagnosed and treated, secondary complications may develop, especially in children. These complications may include rheumatic fever, which can damage the heart, and glomerulonephritis, which affects the kidneys. Because streptococcal infections are routinely diagnosed and treated, these complications are rare in the United States now, but they do still occur.
A rapid strep test and/or a throat culture is used to diagnose group A streptococci as the cause of symptoms and allows the health practitioner to prescribe the proper antibiotics for treatment. (Read more on the "The Test" tab.)
How is the sample collected for testing?
A health practitioner uses a tongue depressor to hold down a person's tongue and then inserts a special swab into the mouth and rubs it against the back of the throat and tonsils. The swab may be used to do a rapid strep test in a doctor's office or clinic, or it may be sent to a laboratory. A second swab may be collected along with the first one. This extra sample is used to perform a throat culture as a follow-up test, when necessary.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed. The test should be performed before antibiotics are prescribed.
- How long does treatment for strep throat usually last?
Ten to 14 days, depending on the antibiotic prescribed. Although your symptoms may improve or disappear before you have taken all of your antibiotics, you should complete your full course of treatment by taking all of the pills that were prescribed.
- How long should I stay away from other people if I have a positive test result?
You should complete at least 24 hours of antibiotics before close contact with others.
- When can my child go back to school?
Usually after one full day of therapy and absence of significant fever; however, a few small studies have found that children may return to school as soon as 12 hours after taking their first dose of antibiotic provided they no longer have a fever and their symptoms have improved.
- If one child in my family has strep throat, is everyone going to get sick?
Other family members, including adults, can be infected by the bacteria. The healthcare provider may test all family members who have sore throats and may test children under the age of 3. In most cases, it is not necessary to test other family members who do not have symptoms.
- What can I do to prevent the spread of strep throat?
The spread of strep throat can be reduced by good hand washing, especially after coughing and sneezing and before preparing food or eating. People with sore throats should be seen by a healthcare provider who can perform tests to find out whether the illness is strep throat. If the test result shows strep throat, the person should stay home from work, school, or daycare until 24 hours after taking an antibiotic.
- I've had strep throat before and was treated with antibiotics. Can I get it again?
Yes. Although antibodies may protect those who have had previous strep infections, there are so many different strains of this organism that being immune to all of them is unlikely. Therefore, someone could potentially get strep throat again and again. The best way to decrease the risk of transmission to others is to minimize close contact with others when ill and wash hands often and thoroughly with soap and water or alcohol-based hand scrub.
- What is an ASO test and how is it used to detect a strep infection?
Antistreptolysin O (ASO) 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 ordered now compared to thirty years ago. For an acute strep throat infection, the ASO test is not helpful; the rapid strep test or throat culture should be used. However, if a health practitioner 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 or glomerulonephritis, which occurs weeks after a strep throat infection when the rapid strep and throat culture would no longer be positive.
- Do other group A streptococous infections occur?
Group A streptococcus can also cause infections that occur separately from strep throat, such as impetigo and, rarely, more invasive conditions such as toxic shock syndrome or necrotizing fasciitis (the so-called "flesh-eating bacteria").
- Are there other types of streptococci that can cause a sore throat?
Group C and group G streptococci, normally found in animals, can occasionally cause pharyngitis in humans. However, these bacteria do not pose a risk for the serious secondary complications associated with group A streptococci. Antibiotic treatment for group A streptococci will be effective against these organisms as well.
© 2017 American Association for Clinical Chemistry, republished from Lab Tests Online.*
Descriptions of clinical laboratory tests were originally prepared for use on Lab Tests Online, an award-winning patient education website on clinical laboratory testing. Lab Tests Online is produced by the American Association for Clinical Chemistry (AACC), a global scientific and medical professional organization dedicated to clinical laboratory science and its application to healthcare. The Lab Tests Online website is developed in collaboration with other laboratory professional societies and is funded in part through corporate sponsorships.