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RADT Vs RT-PCR

  • 20 Jul 2020
  • 5 min read

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Recently, New Delhi's Covid-19 testing strategy has become controversial due to the low level of RT-PCR (Reverse Transcription Polymerase Chain Reaction) re-testing in persons tested negative in RADT (Rapid Antigen Detection Tests).

Key Points

  • ICMR Guidelines:
    • RADT ought to be used only in containment zones, hotspots, hospital settings and laboratories among those who manifested one or other symptoms of the disease, influenza-like illnesses.
    • People with comorbidities who were asymptomatic and high-risk contacts of those confirmed positive.
    • Those who tested ‘negative’ and whom clinicians suspected to be harbouring the disease ought to be definitely tested sequentially by RT-PCR to rule out infection and higher chances of false negatives.
    • Those who test positive don’t need a re-test and must be considered positive.
  • Testing in New Delhi:
    • From 18th June - 16th July, it has conducted 3,05,820 RADT. Of these, 2,85,225 tests came ‘negative’ and out of them only, 1,670 were chosen for re-test by RT-PCR and 262 of these were confirmed positive.
    • Only 1 in 200 of those who tested negative in an antigen test to detect possible coronavirus cases were re-tested, which is against the given guidelines of ICMR.
    • Of those re-tested with RT-PCR, around 15% tested positive, which is higher than the RADT positive results i.e. 6%.
  • Arguments for Low Re-tests:
    • Re-testing everyone would defeat the purpose of having another (rapid antigen) test.
    • The RT-PCR test takes a minimum of 2-5 hours including the time taken for sample transportation. This limits the widespread use of the test and also impedes quick augmentation of testing capacity in various containment zones and hospital settings.
      • In RADT, the maximum duration for interpreting a positive or negative test is 30 minutes, thus a quicker complement to the standard RT-PCR tests.
  • Arguments Against:
    • The consequence of indiscriminately deploying antigen tests would mean expanding the number of tests and presenting a lower positivity rate while not necessarily being able to reliably establish the extent of the spread of the coronavirus in the population.
    • A low level of re-testing with RT-PCR in persons who are testing antigen negative will underestimate the cases and make the tracking inaccurate.

RADT

  • 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.
  • 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..
    • In a reliable rapid antigen detection test, the maximum duration for interpreting a positive or negative test is 30 minutes.

RT-PCR Test

  • Kary Mullis, the American biochemist invented the PCR technique. He was awarded the Nobel Prize for Chemistry in 1993.
  • Under this, copies of a segment of DNA (deoxyribonucleic acid) are created using an enzyme called Polymerase.
    • The ‘chain reaction’ signifies how the DNA fragments are copied, exponentially — one is copied into two, the two are copied into four, and so on.
  • A fluorescent DNA binding dye called the “probe” is added to DNA, which shows the presence of the virus on a fluorometer.
  • However, coronavirus is made of RNA (ribonucleic acid).
  • Therefore to detect coronavirus, RNA is converted into DNA using a technique called reverse transcription.
    • A ‘reverse transcriptase’ enzyme converts the RNA into DNA.
  • Copies of the DNA are then made and amplified.

Source: TH

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