this mucus sample is then exposed to a reagent containing antibodies that will bind specifically to a GAS antigen. A positive result is signified by a certain visible reaction. There are three major types of RST: First, a latex fixation test, which was developed in the 1980s and is largely obsolete. It employs latex beads covered with antigens that will visibly agglutinate around GAS antibodies if these are present. Second, a lateral flow test, which is currently the most widely used RST. The sample is applied to a strip of nitrocellulose film and, if GAS antigens are present, these will migrate along the film to form a visible line of antigen bound to labeled antibodies. Third, optical immunoassay is the newest and more expensive test. It involves mixing the sample with labeled antibodies and then with a special substrate on a film which changes colour to signal the presence or absence of GAS antigen. Interpretation The specificity of RSTs for the presence of GAS is at least 95%, with some studies finding close to 100% specificity. Therefore, if the test result is positive, the presence of GAS is highly likely. However, 5% to 20% of individuals carry GAS in their throats without symptomatic infection, so the presence of GAS in an individual with pharyngitis does not prove that this organism is responsible for the infection. The sensitivity of lateral flow RSTs is somewhat low at 65% to 80%. Therefore, a negative result from such a test cannot be used to exclude GAS pharyngitis, a considerable disadvantage compared with microbial culture, which has a sensitivity of 90% to 95%. However, optical immunoassay RSTs have been found to have a much higher sensitivity of 94%. Although an RST cannot distinguish GAS infection from asymptomatic carriage of the organism, most authorities recommend antibiotic treatment in the event of a positive RST result from a person with a sore throat. US guidelines recommend following up a negative result with a microbial culture, whereas European guidelines suggest relying on the negative RST. See also Screening (medicine) Diagnostic test References Medical tests Medical terminology