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With COVID-19 pandemic, people across the world are becoming increasingly familiar with scientific terms. One such term that now becomes part of the public vocabulary is ‘Ct value’ which is indicated in RT-PCR tests which help to determine whether a patient is COVID-19 positive or not.
Today’s IE’s report refers to how the Maharashtra government had recently sought clarity from the Indian Council of Medical Research (ICMR) to understand whether it is advisable to go forward with treating a person as Covid-negative if the Ct value exceeds 24 and the person seems to be asymptomatic.
Given that the ICMR documents indicated different Ct values and divergent views were emerging on the same, the state had requested for clarity on the course of action to be taken with regard to the Ct value.
After taking inputs from various virology labs, the ICMR DG responded to the state’s query and wrote to its Health Secretary. According to the IE report, ICMR had advised that all those patients with Ct value less than 35 may be considered as positive and those with Ct value over 35 may be considered as negative.
RT-PCR tests: What is Ct value?
When taking the RT-PCR test, Ct value is taken into consideration. A Ct value refers to the number of cycles after which the virus can be detected. If a higher number of cycles is required, the implication is that the virus was undetected when the number of cycles was on the lower side. The lower the CT value, the higher is the viral load as it also means that the virus has been spotted after fewer cycles.
RT-PCR tests: Why is CT-value important?
According to the ICMR, a patient is considered Covid-positive if the Ct value is under 35. Simply put, if the virus is detected after 35 cycles or prior to it, a patient is considered as positive.
Suppose this benchmark is lowered to 24 — which is the value mentioned by Maharashtra in its letter to ICMR while seeking clarity on the matter — it would imply that Ct values in the 25-34 range would not be considered as positive.
Therefore, a benchmark of 35 means that more patients would be treated as being COVID-19 positive than if the benchmark were 24. Lowering the Ct threshold parameter may lead to missing several infectious persons, according to ICMR.
The IE report cites leading virologist Dr Shahid Jameel, Director of Trivedi School of Biosciences, Ashoka University as having stated that one can consider Ct value as a measure of transmission potential.
Significance of the ICMR threshold of 35
Worldwide, the accepted Ct value cut off for Covid-19 is between 35 and 40, subject to instructions from respective manufacturers of the testing equipment. The ICMR has indicated the Ct value of 35 based on input that have been taken from several virology labs.
While the ICMR has issued no new advisory, it has informed the Maharashtra government that it is not advisable to use a lower cycle threshold parameter. The danger is that such a move would lead to missing several infectious persons and increase disease transmission, according to Dr Balram Bhargava, ICMR’s DG, who has been quoted by IE in its report.
Correlation between a Ct value and severity of disease
Citing Dr Parikshit Prayag, consultant for infectious diseases at Deenanath Mangeshkar Hospital in Pune, IE’s report has quoted his insights on the same:
1. Ct value does not correlate with severity
2. It correlates only with infectivity.
3. From the infectivity point of view, it can matter, not severity
It is also pertinent to consider that several factors are important while interpreting an RT-PCR test. Also, the RT-PCR test results may further depend on the exact method of specimen collection and the time from the point of infection to its collection and then analysis.