Rapid Antigen Detection Test

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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