3.2.0.1. Rapid antigen tests (Ag-RDTs) Ag-RDTs are designed to detect the S and N proteins of SARS-CoV-2 in upper respiratory tract specimens collected from COVID-19 patients. The most commonly used Ag-RDT for SARS-CoV-2 diagnosis uses the lateral flow immunoassay (LFIA) principle, as summarized in Figure 3 A. Briefly [50], antigens in the swab sample are loaded into a well with a sample pad and flow through capillary motion up to the conjugate pad containing SARS-CoV-2 monoclonal antibodies (mAbs) and control rabbit mAbs both tagged with detector molecules, such as colloidal gold nanoparticles (AuNPs). In the event of a positive sample, SARS-CoV-2 antigens link to the bound AuNPs at the conjugation pad forming a complex that moves by capillary action to the test line containing other SARS-CoV-2 antigen-specific mAbs. Here, antigen–antibody complexes become trapped forming a calorimetrically visible line that can be seen by the naked eye or with the help of a detector indicating sample positivity. Subsequently, the buffer containing rabbit mAbs migrates further down to the control line where they are captured by specific anti-rabbit antibodies. If the test is performed accurately, the control line also becomes calorimetrically visible meaning that the test is valid [50] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8559769/

TLDR: The sample is mixed with some fluid and is dropped on a slide primed with two types of monoclonal antibodies. IF those react then a 'stripe' becomes visible.