Coronavirus – Disease, Medical Sciences Involved & Preventive Measures

Understanding the Ct value in a Covid-19 test

Note4Students

From UPSC perspective, the following things are important :

Prelims level : RT-PCR Test

Mains level : Paper 2- Ct value in RT-PCR test

Recently, the Maharashtra government sought clarity from the Indian Council of Medical Research (ICMR) on the threshold Ct value to treat a person Covid-negative.

What is Ct value

  • Short for cycle threshold, Ct is a value that emerges during RT-PCR tests.
  • In an RT-PCR test, RNA is extracted from the swab collected from the patient.
  • It is then converted into DNA, which is then amplified.
  • Amplification refers to the process of creating multiple copies of the genetic material — in this case, DNA.
  • Amplification takes place through a series of cycles — one copy becomes two, two becomes four, and so on.
  • Put simply, the Ct value refers to the number of cycles after which the virus can be detected.
  • The lower the Ct value, the higher the viral load — because the virus has been spotted after fewer cycles.

Why Ct value is important

  • According to the ICMR, a patient is considered Covid-positive if the Ct value is below 35.
  • If the benchmark were to be lowered to 24 — the value mentioned in Maharashtra’s letter — it would mean that Ct values in the range 25-34 would not be considered positive.
  • A benchmark of 35, therefore, means that more patients would be considered positive than we would get if the benchmark were 24.
  • The ICMR has said lowering Ct threshold parameter may lead to missing several infectious persons.

Does Ct score indicate the severity of disease

  • A small study published in the Indian Journal of Medical Microbiology in January this year found that there was no correlation between Ct values and severity of disease or mortality in patients with Covid-19 disease.
  • It found that the time since the onset of symptoms has a stronger relationship with Ct values as compared to the severity of the disease.
  • The Ct value tells us about the viral load in the throat and not in the lungs.
  • The Ct value does not correlate with severity – only with infectivity.
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