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.

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
• Rash
• 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:

• Cough
• 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.

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.

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.