Rapid strep test

The rapid strep test (RST) is a rapid antigen detection test (RADT) that is widely used clinically to aid in the diagnosis of bacterial pharyngitis caused by group A strep (GAS), sometimes called strep throat.There are several types of rapid strep tests in use, each using a different technique. However, they both work by detecting the presence of GAS in a person's throat by reacting to GAS-specific antigens on throat swabs.

Medical uses 

A rapid strep test can help clinicians decide whether to prescribe antibiotics to people with pharyngitis, a common throat infection.Viral infections are responsible for most pharyngitis, but a significant proportion (20% to 40% in children and 5% to 15% in adults) are caused by bacterial infections.Symptoms of viral and bacterial infections can be indistinguishable, but only bacterial pharyngitis is effectively treated with antibiotics.Since the main cause of bacterial pharyngitis is gas, the presence of this organism in a person's throat may be considered necessary for prescribing antibiotics.GAS pharyngitis is a self-limiting infection that usually resolves within a week without medication.However, antibiotics may shorten the duration and severity of the disease and reduce the risk of certain rare but serious complications, including rheumatic heart disease.RSTs may also have a public health benefit. In addition to undesirable side-effects in the individual, inappropriate antibiotic use is thought to contribute to the development of drug-resistant strains of bacteria.By helping to identify bacterial infection, RSTs may help to limit the use of antibiotics in viral illnesses, where they are not beneficial.Some clinical guidelines recommend the use of RSTs in people with pharyngitis, but others do not.US guidelines are more consistently in favor of their use than their European equivalents.The use of RSTs may be most beneficial in the third world, where the complications of streptococcal infection are most prevalent, but their use in these regions has not been well studied.Throat swab microbial culture is a reliable and economical alternative to RST with high sensitivity and specificity.However, culture requires special equipment and typically takes 48 hours to give results, whereas RST can give results in minutes.

Procedure

The patient's throat is first swabbed to collect a sample of mucus. In most RSTs, this mucus sample is then exposed to a reagent containing antibodies that specifically bind the GAS antigen.A positive result is indicated by some visible reaction.There are three main types of RST: First, the latex fixation test, which was developed in the 1980s and is largely outdated.It uses latex beads coated with antigens that, if present, will visibly clump around the GAS antibody.The second is the lateral flow test, which is currently the most widely used RST.The sample is applied to a nitrocellulose membrane and, if present, GAS antigens will migrate along the membrane to form a visible line of antigen bound to the labeled antibody.Third, optical immunoassays are the newest and more expensive tests.It involves mixing the sample with labeled antibodies and then with a special substrate on a membrane that changes color to indicate the presence of the GAS antigen.