GS PAPER II: Issues relating to development and management of Social Sector/Services relating to Health, Education, Human Resources.

Context: The cycle threshold value emerges in RT-PCR tests for the coronavirus, and determines whether a person is positive for Covid-19 or not.

• An RT-PCR test detects the virus; Ct value sets a benchmark for whether a person is to be counted as positive.
• ICMR has taken inputs from virology laboratories across the country to arrive at a single Ct value cut-off.
• 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.

What is Ct value?

• Short for cycle threshold, Ct is a value that emerges during RT-PCR tests, the gold standard for detection of the SARS-CoV-2 coronavirus.
• 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. This improves the ability of the test to detect the presence of 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 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.

Why is it important?

• According to the ICMR, a patient is considered Covid-positive if the Ct value is below 35.
• In other words, if the virus is detectable after 35 cycles or earlier, then the patient is considered positive.

What is the significance of the ICMR threshold of 35?

• Globally, the accepted cut-off for Ct value for Covid-19 ranges between 35 and 40, depending on instructions from the respective manufacturers of testing equipment.
• The ICMR has arrived at the Ct value of 35 based on laboratory experiences and inputs taken from several virology labs.
• There was no new advisory, but ICMR informed the Maharashtra government that it was not advisable to use a lower cycle threshold parameter as it would lead to missing several infectious persons and increase disease transmission.

Is there any correlation between a Ct value and severity of disease?

• Although Ct value is inversely correlated with viral load, It does not have any bearing on the severity of the disease.
• A patient can have a low Ct value, which means her viral load is high enough to be detected rapidly, but she may still be asymptomatic.
• 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.

Does a high Ct value always mean a low viral load?

• Many factors are important in interpreting an RT-PCR test, and the results may also depend on the method of specimen collection and time from infection to collection and to analysis.
• An ICMR advisory in August last year noted that Ct values depend on how the sample has been collected.
• A poorly collected sample may reflect inappropriate Ct values.
• Besides, Ct values are also determined by the technical competence of the person performing the test, calibration of the equipment, and the analytical skills of the interpreters.
• Again, Ct values may differ between nasal and oropharyngeal specimens collected from the same individual.
• The temperature of transportation, as well as the time taken from collection to receipt in the lab, can also adversely impact Ct values.

https://indianexpress.com/article/explained/explained-the-ct-value-in-a-covid-test-7291682/

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