A COVID-19 Antigen (Ag) Test, also known as a Rapid Antigen Test (RAT) or Lateral Flow Test (LFT), detects specific proteins (antigens) from the SARS-CoV-2 virus, giving fast results (5-30 mins) for active infections, often used at home or in clinics for quick screening, though typically less sensitive than PCR tests for very early or low viral loads