CT value impacting Covid-19 fatality: A ‘viral’ myth

With the pandemic shaking India with its second wave, new medical terms coming out every other day are creating speculation and unnecessary panic.

The latest of such cases involves the rumour that a patient's CT value is directly linked to how fatal the infection is.

Before debunking the viral myth, it is important to understand what ‘CT value’ means.

Read more: One in four people experience mild side effects from Covishield Covid-19 vaccine: Lancet study

When a person undergoes an RT-PCR test, CT (Cycle Threshold) is a value that is detected. RNA is collected from the swab during the test and then converted into DNA. DNA is then amplified, which simply means that multiple copies of the DNA are produced – a method used to detect the presence of the virus. It takes multiples ‘amplification’ cycles for a detectable amount of virus to be produced.

The current standard for a person to be declared Covid positive is if their CT value is below 35, according to the Indian Council of Medical Research (ICMR). Therefore, if it takes 35 or lesser cycles for the virus to be detected, the person becomes 'Covid positive'. Having a benchmark of ’35 CT value’ means more people testing positive than if the requirement was brought down to 24 cycles - the value mentioned in a letter sent by the Maharashtra government.

Lowering the Cycle Threshold, the ICMR explains, could lead to many infectious people going over the radar.

Virologist Dr Shahid Jameel told The Indian Express that CT value is a measure of ‘transmission potential’. “If there is more virus in my throat and nose, I will transmit it better,” he explained.

A study by the ICMR found that a person’s CT value is not related to the severity of Covid-19. The CT value, despite being inversely correlated with the viral load, has no relation to the severity of infection. An asymptomatic person could have a low CT value (high viral load).

Dr Parikshit Prayag, a consultant for infectious diseases at the Deenanath Mangeshkar Hospital in Pune, told The Indian Express that CT value shows us the viral load present in a person’s throat and not their lungs.

“The CT value does not correlate with severity – only with infectivity. In the first report, I do not really look at the CT value, but for follow-up of patients in the hospital, I do consider the CT value as then I can decide whether to transfer the patient to the non-Covid building or not. From the infectivity point of view, it can matter, not severity”, he said.