Rapid Antigen Test

  • Recently, the Indian Council of Medical Research (ICMR) approved the use of rapid antigen detection test in containment zones and healthcare settings in combination with the RT-PCR test.
  • The ICMR has allowed the use of antigen detection kits developed by the South Korean company S D Biosensor, which has a manufacturing unit in Manesar.
  • The kit, commercially called Standard Q COVID-19 Ag detection kit, comes with a with an inbuilt Covid antigen test device, viral extraction tube with viral lysis buffer and sterile swab for sample collection.

Rapid Antigen Detection Test for Covid-19

  • It is a test on swabbed nasal samples that detects antigens (foreign substances that induce an immune response in the body) that are found on or within the SARS-CoV-2 virus.
  • It is a point-of-care test, performed outside the conventional laboratory setting, and is used to quickly obtain a diagnostic result.
  • The test has to be performed onsite under strict medical supervision and maintaining the kit temperature between 2° and 30°C.

Rapid Antigen Test

  • It 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-
  • Rapid strep tests (for streptococcal antigens)
  • Rapid influenza diagnostic tests (RIDTs) (for influenza virus antigens)
  • Malaria antigen detection tests (for Plasmodium antigens)

Antigens

An antigen is any substance that causes your immune system to produce antibodies against it.

An antigen may be a substance from the environment, such as chemicals, bacteria, viruses, or pollen.

Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of adaptive immune system toward the antigens of the pathogen invading that recipient.

Difference from RT-PCR Test

  • Like RT-PCR, the rapid antigen detection test too seeks to detect the virus rather than the antibodies produced by the body.
  • While the mechanism is different, the most significant difference between the two is time.
  • RT-PCR test takes a minimum of 2-5 hours including the time taken for sample transportation. These specifications limit the widespread use of the RT-PCR test and also impede quick augmentation of testing capacity in various containment zones and hospital settings.
  • In a reliable rapid antigen detection test, the maximum duration for interpreting a positive or negative test is 30 minutes.

Accuracy of Rapid Antigen Test

  • The results revealed that the kit had a very high specificity, or the ability to detect true negatives, ranging between 99.3% and 100% at the two sites.
  • The sensitivity of the test, or its ability to detect true positives, ranged between 50.6% and 84%, depending upon the viral load of the patient.
  • The higher the ability to detect true negatives, the more reliable is any positive result.
  • A negative test result may occur if the level of an extracted antigen in a specimen is below the sensitivity of the test or if a poor quality specimen is obtained
  • A negative result may also occur if the concentration of antigen in a specimen is below the detection limit of the test or if the specimen.
  • Also, children tend to shed the virus for longer periods than adults, which may result in differences in sensitivity between adults and children.

Limitations

  • 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.
  • Further, once the sample is collected in the extraction buffer, it is stable only for one hour.
  • Therefore, the antigen test needs to be conducted at the site of sample collection in the healthcare setting.

Source : Civil Services Chronicle Online, June, 2020