IntroductionRapid antigen test for COVID 19 (also called RAD detects the presence of a specific viral antigen, which implies the current coronavirus infection in the body RAT is a type of rapid and simple test which gives visual results and can be seen with the naked eye. Most antigen tests are relatively inexpensive, and most can be used at the time of clinical check-up itself.RAT is primarily used in screening and diagnosing COVID-19. Most of the currently permitted tests produce results in approximately 15-30 minutes.While other diagnostic tests for COVID-19, known as RT- PCR tests, look for the genetic material from the virus, the antigen test looks for the molecules on the surface of the virus.Rapid antigen tests can be used to diagnose active infection in symptomatic people. Asymptomatic individuals may also opt for this test if they have come in close contact with an individual with COVID-19, engaged in long-term care facilities, correctional and detention facilities, homeless shelters) or to screen hospitalised people in high-prevalence areas. These may also be performed before surgical procedures or aerosol-generating procedures and before receiving immune suppressants.correctional and detention facilities, homeless shelters) or to screen hospitalised people in high-prevalence areas. These may also be performed before surgical procedures or aerosol-generating procedures and before receiving immune suppressants.How the test is doneAntigen test does not require any special preparation. Whether performed in a medical setting or at home, the test sample is obtained by inserting a cotton-tipped swab into the nostril. Once the swab is inserted, it is usually rotated and a sample is often taken from both nostrils. It usually takes under a minute to take the test.The insertion of the swab into the nose may give an unpleasant sensation, especially when a sample is being taken from the nasopharynx (behind the nose) region.There may be a temporary need to cough or sneeze or the eyes may water. It is rare to have any lasting effects after the test sample is taken.ResultsIn case of negative results, symptomatic individuals may be subsequently recommended RT- PCR to rule out infection. In the meantime, they are advised to follow home isolation and proper treatment as negative reports on RAT may not be true negative in some cases.A positive result should be considered as true positive and does not need reconfirmation by RT-PCR testing.The specificity of antigen tests is generally as high as most viral diagnostic tests, which means that false-positive test results are unlikely when an antigen test is performed according to the instructions.AdvantagesRAT advantages over RT-PCR due to its rapid results, easy procedure and no requirement of instrument. It allows access to testing in areas where laboratory testing is not available. Molecular tests like the reverse transcription polymerase chain reaction (RT-PCR or just PCR) are considered the gold standard for diagnosis but when those tests are unavailable, an antigen test could be used for diagnosis in people who have symptoms consistent with COVID-19.The sensitivity of antigen tests varies but is generally lower than most laboratory-based viral diagnostic tests. There is a higher chance of false negatives, so negative results do not disregard a chance of infection. Dr Anuja Mahajan is a consultantpathologist.