A rapid antigen detection test (RADT) that is widely used in clinics to assist in the diagnosis of bacterial pharyngitis caused by group A streptococci (GAS), termed strep throat.
Several types of rapid strep test in use employ technology to detect the presence of GAS in the throat of a person by responding to GAS-specific antigens on a throat swab.
The test may assist in deciding whether to prescribe an antibiotic to a person with pharyngitis.
While viral infections are responsible for the majority of pharyngitis, a significant proportion (20% to 40% in children and 5% to 15% in adults) is caused by bacterial infection.
The major cause of bacterial pharyngitis is GAS, the presence of this organism in a person’s throat may be seen as a necessary condition for prescribing antibiotics.
GAS pharyngitis is a self-limiting infection that will usually resolve within a week without medication.
Antibiotics may reduce the length and severity of GAS pharyngitis and reduce the risk of certain rare but serious complications.
The rapid streptococcus test may help to limit the use of antibiotics in viral illnesses, where they are not beneficial.
Microbial culture from a throat swab is a reliable and affordable alternative RST and has high sensitivity and specificity.
A culture requires special facilities and usually takes 48 hours to give a result, whereas an RST can give a result within several minutes.
The throat is first swabbed to collect a sample of mucus. which is then exposed to a reagent containing antibodies that will bind specifically to a GAS antigen.
A positive result is recognized by a visible reaction.
There are three major types of RST: a latex fixation test, a lateral flow test, which is currently the most widely used RST, and an optical immunoassay.
The specificity of the test for the presence of GAS is at least 95%, with close to 100% specificity.
The presence of GAS in an individual with pharyngitis does not prove that this organism is responsible for the infection, because 5% to 20% of individuals carry GAS in their throats.
Lateral flow RSTs have a lower sensitivity at 65% to 80%.
Optical immunoassay RSTs have been found to have a sensitivity of 94%.
Microbial culture has a sensitivity of 90% to 95%.
Most recommend antibiotic treatment in the event of a positive RST result from a person with a sore throat.