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Ct Value: Covid-19 Test

  • 28 Apr 2021
  • 3 min read

Why in News

The ICMR (Indian Council of medical Research) has decided a single Ct value cut-off for determining whether a patient is positive for Covid-19.

  • Ct is a value that emerges during RT-PCR tests. All patients with a Ct value less than 35 may be considered as positive while those with a Ct value above 35 may be considered as negative.

Key Points

  • RT-PCR Tests:
    • In an RT-PCR (Reverse Transcription Polymerase Chain Reaction) test, RNA (Ribonucleic acid) is extracted from the swab collected from the patient. It is then converted into DNA (Deoxyribonucleic acid), which is then amplified.
    • Amplification refers to the process of creating multiple copies of the genetic material - in this case, DNA.
      • This improves the ability of the test to detect the presence of the virus.
    • Amplification takes place through a series of cycles—one copy becomes two, two becomes four, and so on—and it is after multiple cycles that a detectable amount of virus is produced.
  • Ct Value:
    • Ct is short for ‘Cycle Threshold’.
    • The Ct value refers to the number of cycles after which the virus can be detected.
    • If a higher number of cycles is required, it implies that the virus went undetected when the number of cycles was lower.
    • The lower the Ct value, the higher the viral load-because the virus has been spotted after fewer cycles.
    • It has been found that the time since the onset of symptoms has a stronger relationship with Ct values as compared to the severity of the disease.
  • Viral Load:
    • It refers to the amount of genetic material, commonly RNA, of a virus present in an infected person’s blood.
    • This is expressed as the total number of viral particles present in each millilitre of blood.
    • A higher viral load in the blood means that the virus is replicating and the infection is progressing.
    • An infected person with a high viral load is more likely to shed more virus particles, in the process known as “viral shedding”.

Source: IE

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