Union Health Minister Harsh Vardhan said today that  ICMR has switched protocol and now testing for COVID-19 from today onwards would be as per the new Rapid Antigen methodology. "This technique will be faster and cheaper, 169 testing centers set up in the National Capital. Priority for the supply of kits will be given to Delhi," he added.

A rapid antigen test (RAT) is a rapid diagnostic test suitable for point-of-care testing that directly detects the presence or absence of an antigen. This distinguishes it from other medical tests that detect antibodies (antibody tests) or nucleic acid (nucleic acid tests), of either laboratory or point of care types.

Common examples of RATs or RADTs include:

Rapid strep tests (for streptococcal antigens)[1]
Rapid influenza diagnostic tests (RIDTs) (for influenza virus antigens)
Malaria antigen detection tests (for Plasmodium antigens)

Antigen tests and antibody tests are often immunoassays (IAs) of one kind or another, such as dipstick IAs or fluorescence immunoassays. While RAT is an immunochromatographic assay that 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, 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 an 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.

Recently FDA also invokes EUA to legitimise  Rapid Antigen Test, for COVID-19 detection. According to the FDA , each category of the diagnostic test has its own unique role in the fight against this virus. PCR tests can be incredibly accurate, but running the tests and analyzing the results can take time. One of the main advantages of an antigen test is the speed of the test, which can provide results in minutes. However, antigen tests may not detect all active infections, as they do not work the same way as a PCR test. Antigen tests are very specific for the virus but are not as sensitive as molecular PCR tests. This means that positive results from antigen tests are highly accurate, but there is a higher chance of false negatives, so negative results do not rule out infection. With this in mind, negative results from an antigen test may need to be confirmed with a PCR test prior to making treatment decisions or to prevent the possible spread of the virus due to a false negative.

Antigen tests are also important in the overall response against COVID-19 as they can generally be produced at a lower cost than PCR tests and once multiple manufacturers enter the market, can potentially scale to test millions of Americans per day due to their simpler design, helping our country better identify infection rates closer to real-time.

This is just the first antigen test to be authorized and we could expect more to follow.