Streptococcal infections: how and why to perform the rapid test

By Cristiano Antonino On Feb 9, 2023

Streptococcal infection: the rapid test is used to check whether a person with a sore throat (pharyngitis) has streptococcal pharyngitis, i.e. an infection of the throat and tonsils caused by the bacterium Streptococcus pyogenes, also called group A β-haemolytic Streptococcus

Many sore throats are of viral origin and may resolve spontaneously within a few days; some people, however, suffer from streptococcal pharyngitis.

It is important to diagnose and treat this type of infection promptly because of the high probability of infection and the development of secondary complications, especially in children.

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The rapid test, also known as rapid antigen detection test (RADT), can detect the presence of specific antigens for group A streptococcus, providing a result in 10-12 minutes

The U.S. Food and Drug Administration (FDA) has recently approved the use of a molecular test that can detect the presence of genetic material belonging to group A streptococcus in pharyngeal swabs in just eight minutes.

A positive rapid test indicates the presence of an infection with group A streptococcus, the bacterium that causes streptococcal pharyngitis.

A negative rapid test, on the other hand, indicates that the person with a sore throat most likely does not have streptococcal pharyngitis although, in children and adolescents, a pharyngeal swab culture test is recommended as confirmation.

It is recommended to consult the paediatrician and perform the rapid test in the case of:

Acute onset sore throat lasting more than a week, or recurrent sore throat;
Reddened (inflamed) throat and/or tonsils with white or yellow plaques or streaks
Fever of 38°C or higher
Skin rashes
Hoarseness lasting more than two weeks
Small red spots on the palate
Difficulty swallowing
Headaches, muscle aches
Nausea or vomiting
Pain and swollen cervical lymph nodes
Blood in saliva and mucus

According to the 2012 guidelines provided by the Infectious Diseases Society of America (IDSA), the test is not recommended if other symptoms attributable to a viral-type infection are also present, such as: Cough, rhinorrhoea (runny nose), mouth ulcers.

Streptococcus, the test is performed by taking an oro-pharyngeal sample and a rapid test is possible

In this case, specific antigens of group A beta-haemolytic Streptococcus are sought, and therefore the test has an absolute specificity and sensitivity of 96 %.

The test is called rapid because the result is available in 10 to 15 minutes.

With the rapid swab it is also possible to detect the presence of both viable and non-viable elements (still present 24/48 hours after the start of antibiotic therapy, although no longer able to develop into colonies when a culture test is performed).

With the rapid or direct test, however, it is not possible to detect microbial agents other than group A beta-haemolytic Streptococcus.