Context: Recently, the Indian Council of Medical Research (ICMR) approved one more kind of test for diagnosis of Covid-19, commercially called Standard Q COVID-19 Ag detection kit.
About the test:
- The rapid antigen detection test is to be used in specified settings, and kits from only one manufacturer have got approval currently.
- 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 kit comes with an inbuilt Covid antigen test device, viral extraction tube with viral lysis buffer and sterile swab for sample collection.
- In India, 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.
- As of now, the kit will be used in containment zones or hotspots and healthcare settings.
Comparison with 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, which limits the widespread use of the RT-PCR test and 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.
How accurate the new test is?
- 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% at the two sites.
- The higher the ability to detect true negatives, the more reliable is any positive result.
- 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.
Is the test a confirmatory one?
- 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.
Limitations of an antigen test’s results:
- According to the US FDA, 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.
- Testing at the site of sample collection: 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.
According to SD Biosensor, children tend to shed the virus for longer periods than adults, which may result in differences in sensitivity between adults and children.