What is Cycle threshold (Ct) Value in a RT-PCR Test?

The Ct Value in a RT-PCR test is cycle Threshold value. This value determines if a patient is COVID-19 positive or not.

Why is it in news?

The State Government of Maharashtra recently asked the Indian Council of Medical Research if it was advisable to treat a person as COVID-19 negative if his Ct value was more than 24.

What is the issue?

The ICMR documents mention different Ct Values. Also, there are divergent views among the National Centre for Disease Control and also among Niti Aayog.

What is the right Ct value for a person to test COVID-19 positive?

The ICMR recently announced that the right Ct value for a person to become COVID-19 positive is 35. If the Ct value of a person is less than 35, he is COVID-19 positive and if it is greater than 35 then he is COVID1-19 negative.

What is Ct value?

  • In a RT-PCR test, the RNA extracted from the swab is converted into DNA. The DNA is then amplified. The amplification is done to create multiple copies of the genetic material. This is done to improve the ability of the test to detect the virus.
  • The amplification takes place in cycles. At first cycle, one copy of the DNA becomes two. In the second cycle two copies become four and so on.
  • The Ct value is the number of cycles after which the virus was detected.
  • If the number of cycles is low, then the viral load is high. Because, the virus was detected in very few cycles.

The significance of the problem

The Maharashtra State Government had asked if the Ct value can be fixed at 24. This means that the persons with Ct values 24-35 will not be considered positive. On the other hand, the bench mark of 35 means that more patients are considered positive. Lowering the threshold may lead to missing infectious persons.

Global Ct value

Globally, the accepted Ct value is between 35 and 40.


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