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Ct value in Covid-19 test

Date: 29 April 2021 Tags: Miscellaneous


The Ct value has been a major term that has been gaining importance in determining whether a patient is positive for Covid-19 in RT-PCR test.



  • The Maharashtra government has sent a request of clarity to ICMR whether to treat an individual with Ct value above 24 and asymptomatic as positive case.

  • The ICMR has come up with a single Ct cut-off value where less that 35 would be treated as a positive case and above 35 will be negative.


Ct value

  • Ct stands for Cycle threshold and is a major indicator of viral infection in patients during RT-PCR tests.

  • Ct value stands for the number of cycles after which the virus can be detected from a given sample.

  • Higher the number of cycle, lower the viral load as virus is not easily detected in earlier number of cycles. Higher the Ct value, lower the viral load.


Process of identification

  • In RT-PCR tests, RNA is extracted from patient swabs and is later converted into DNA through a process known as amplification.

  • In amplification, multiple copies of DNA are made over and over again so that a detectable amount of viral samples can be identified.

  • Amplification takes place in a series of cycles and after multiple cycles it is possible to determine the case of virus.


Relation with severity

  • Ct value does not have direct relation with disease severity. Individuals with lower Ct value may be asymptomatic.

  • There may be many reasons for influencing infection levels such as method of specimen collection and time from infection to collection.

  • Ct value is also dependent on the method of sample collection along with the technical competence of the person performing test.

  • Other influencing factors in Ct value include transportation, nasal or oropharyngeal specimens from a same individual etc.

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