Rapid Antigen Detection Test

Why in news?

The Indian Council of Medical Research (ICMR) approved the rapid antigen detection test for diagnosis of Covid-19.

What is the rapid antigen detection test?

• It is a point-of-care test, performed outside the laboratory setting.
• It is a quick diagnostic test on swabbed nasal samples. It detects antigens that are found on or within the SARS-CoV-2 virus.
• [Antigens are foreign substances that induce an immune response in the body.]
• In India, the ICMR has allowed the use of antigen detection kits developed by the South Korean company S D Biosensor.
• SD Biosensor has a manufacturing unit in Manesar.
• The kit is commercially called Standard Q COVID-19 Ag detection kit.
• It comes with an inbuilt Covid antigen test device, viral extraction tube with viral lysis buffer and sterile swab for sample collection.

How is rapid antigen detection test different from RT-PCR test?

• RT-PCR is currently the gold standard frontline test for the diagnosis of Covid-19.
• 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.
• As the ICMR has pointed out, the 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.
• It also impedes 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.

Why has been only the kit by the company SD Biosensor allowed?

• Very few reliable antigen detection kits for Covid-19 diagnosis are available worldwide.
• The US FDA authorised the first antigen test, with US-based Quidel’s antigen kit called the Sofia 2 SARS Antigen FIA.
• In India, the ICMR conducted an independent two-site evaluation of the SD Biosensor kit, at ICMR and AIIMS.
• The results revealed that the kit had a very high specificity, or the ability to detect true negatives, ranging between 99.3% and 100%.
• 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.
• Having allowed SD Biosensor to market its kit commercially, ICMR has also asked other manufacturers/developers who have antigen detection assays to come forward for validation.

Where will the test be used?

• As of now, the kit will be used in containment zones or hotspots and healthcare settings.
• Test will be performed onsite under strict medical supervision and maintaining the kit temperature between 2° and 30°C.
• In containment zones - The test can be conducted on all symptomatic influenza-like illnesses.
• Asymptomatic direct and high-risk contacts with co-morbidities of a confirmed case are to be tested once between day 5 and 10 of coming into contact.
• In healthcare settings - It can be used in three categories.
  1. In all persons presenting influenza-like symptoms in a healthcare setting and suspected of having Covid-19 infection;
  2. In asymptomatic patients who are hospitalised or seeking hospitalisation, in the high-risk groups and
  3. In asymptomatic patients undergoing aerosol-generating surgical procedures /non-surgical interventions.

Is the test a confirmatory one for diagnosis of Covid-19?

• According to the ICMR guidelines, if the test shows a positive result, it should be considered as true positive, and does not need reconfirmation.
• However, those who test negative in the rapid antigen test should then be tested by RT-PCR to rule out infection.

What are the limitations of an antigen test’s results?

• When it gave emergency authorisation for the first antigen kit, the US FDA pointed out that antigen tests are very specific for the virus.
• But it also said that they are not as sensitive as molecular PCR tests.
• Also, the ICMR has said that 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.
• 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 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.

Source: The Indian Express