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Antigen tests are immunoassays that are frequently used to rapidly diagnose infectious diseases. They detect the presence of proteins from a specific infectious agent.
What is it used for?
The antigen tests use the lateral flow method (rapid immunochromatographic test strips), similar to those used in pregnancy tests, which lets a person know very quickly whether or not they are positive for SARS-CoV-2 and therefore a confirmed case. This means that isolation measures can be implemented quickly when needed. However, they are less sensitive than molecular tests, like PCR, for example. A negative result does not therefore rule out the presence of infection and, in these cases, if there is a high suspicion of infection, other diagnostic tests must be performed.
How is it done?
The test uses the lateral flow method, or immunochromatography. A sample from a nasopharyngeal swab can be used, although devices have also been developed to test other types of samples, including saliva.
The sample is applied to the device, which absorbs through capillary action, and it is then mixed with a conjugate that flows through a coated membrane. The conjugate is a mixture of substances containing antibodies specific for the surface proteins of the SARS-CoV-2 virus. If the result is positive a coloured band appears, indicating that the antibodies have bound to the SARS-CoV-2 proteins present in the saliva sample. If no coloured band appears on the test line, the result is considered negative, but this does not rule out infection.