Rapid strep test and medical uses

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 use

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.RST may also have public health benefits.In addition to causing adverse side effects on individuals, the inappropriate use of antibiotics is thought to promote the development of drug-resistant bacterial strains. By helping to identify bacterial infections,RST may help limit the use of antibiotics in viral diseases where they are not beneficial.Some clinical guidelines recommend the use of RST in patients with pharyngitis, but others do not.US guidelines support their use more consistently than European guidelines.RST use may be most beneficial in the third world, where complications of streptococcal infection are most common, but their use in these areas 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.

Interpretation

RST is at least 95% specific for the presence of GAS,some studies have found it to be close to 100%.Therefore, if the test is positive, GAS is highly likely to be present.However, between 5% and 20% of people with GAS in the throat do not have symptoms of infection, so the presence of GAS in a patient with pharyngitis does not prove that this organism is the cause of the infection.Lateral flow RST has a slightly lower sensitivity of 65% to 80%.Therefore, a negative result of this test cannot be used to rule out GAS pharyngitis, which is a considerable disadvantage compared with microbial culture, which has a sensitivity of 90% to 95%.However, the optical immunoassay RST has been found to have a higher sensitivity of 94%.Although the RST cannot differentiate GAS infection from asymptomatic carriage, most authorities recommend antibiotic therapy in the setting of a positive RST result in patients with sore throat.U.S. guidelines recommend microbiological culture for negative results,while European guidelines recommend relying on negative RST.