Rapid antigen test
A rapid antigen test (RAT), sometimes called a rapid antigen detection test (RADT), antigen rapid test (ART), or loosely just a rapid test, is a rapid diagnostic test suitable for point-of-care testing that directly detects the presence or absence of an antigen. Such tests are a type of lateral flow test that detect antigens, distinguishing them from other medical tests that detect antibodies (antibody tests) or nucleic acid (nucleic acid tests), of either laboratory or point-of-care types. Rapid tests generally give a result in 5 to 30 minutes, require minimal training or infrastructure , and have significant cost advantages. Rapid antigen tests for the detection of SARS-CoV-2, the virus that causes COVID-19, have been commonly used during the COVID-19 pandemic. For many years, an early and major class of RATs—the rapid strep tests for streptococci—were so often the referent when RATs or RADTs were mentioned that the two latter terms were often loosely treated as synonymous with those; today, more precise usage is advisable, as other major classes of RATs are just as important. RATs are based on the principle of antigen-antibody interaction. Antibodies directed against the SAR-CoV-2 virus  antigens, which are proteins on the surface of the virus, will detect the virus in the nasopharyngeal sample. A positive result with an antigen test will need to be confirmed by an RT-qPCR test. Uses. Common examples of RATs or RADTs include: COVID-19 rapid antigen tests. Rapid antigen tests for COVID-19 are one of the most useful application of these tests. Often called lateral flow tests, they have provided global governments with several benefits. They are quick to implement with minimal training, offered significant cost advantages, costing a fraction of existing forms of PCR testing and give users a  result within 5–30 minutes. Rapid antigen tests have found their best use as part of mass testing or population-wide screening approaches. They are successful in these approaches because in addition to the aforementioned benefits, they identify individuals who are the most infectious and could potentially spread the virus to a large number of other people. This differs slightly from other forms of COVID-19 tests such as PCR that are generally seen to be a useful test for individuals. As early as February 2021, the US Department of State considered the antigen test suitable for entry to the country. In  Canada, although the antigen test appeared to be no route to entry in January 2021, Health Canada in August 2021 made available subsidized at no cost rapid antigen tests "to more small and medium-sized organizations through new pharmacy partners". Scientific basis and underlying biology. Antigen tests and antibody tests are often immunoassays (IAs) of one kind or another, such as dipstick IAs or fluorescence immunoassays; however, RAT is an immunochromatographic assay which gives visual results that can be seen with the naked eye. It is considered to be qualitative but a person experienced in RDT testing can easily quantify  the results. Being a screening test, if the sensitivity and specificity are relatively low for the test then the results should be evaluated on the basis of confirmatory tests like PCR testing or western blot. One inherent advantage of an antigen test over an antibody test (such as antibody-detecting rapid HIV tests) is that it can take time for the immune system to develop antibodies after infection begins, but the foreign antigen is present right away. Although any diagnostic test may have false negatives, this latency period can open an especially wide avenue for false negatives in antibody tests , although the particulars depend on which disease and which test are involved. A rapid antigen test typically costs around US$5 to manufacture.