COVID FAQ: What’s CT Value? Does It Tell How Contagious You Are?

Why is everyone is talking about Cycle Threshold value? Here’s what we know.

Why is everyone talking about Cycle Threshold value? Does it really tell how contagious you are?

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A term related to the ‘gold standard’ RT-PCR tests to detect the novel coronavirus has got a lot of attention – the ‘Cycle Threshold’ value.

From a viral video by an Indian cardiologist, to Biocon’s head Kiran Mazumdar Shaw, everyone is talking about Cycle Threshold value and its role in deciding whether you can infect someone else or how contagious you are.

What is CT value?

The Cycle Threshold value is basically the number of cycles needed for the virus to be detected from the person's sample.

The molecules in these samples are very small. Therefore, the RT-PCR test amplifies the samples – and the virus, if detected, is spotted after some cycles.

Will a person with higher CT value have a lesser degree of infection?

The higher the concentration, the lesser number of cycles are needed for the virus to be detected. And if the CT value is higher, it is believed that the concentration is lesser.

For example, if A's CT value is 25 and B's CT value is 15, it means that B is carrying a higher infection load than A.

Reports claim that person with CT value of more than 24 won’t spread infection. Is this true?

Dr Shahid Jameel, Virologist and Director at Trivedi School of Biosciences, Ashoka University falsifies such claims.

He says that if someone with a CT value of 30 is roaming around without a mask, then they are more likely to spread the virus than an infected person with a 22 CT value who stays isolated. In such a situation, fixing the cutoff at 24 can mislead people, he told FIT.

What is ICMR saying about CT value?

The ICMR clearly states that it does not recommend COVID-19 patients to rely on numerical CT values to determine infectiousness and patient management protocols.

Does this mean people with higher CT value should still quarantine?

Yes, says ICMR. Severity of COVID-19 disease largely depends on host factors besides the viral load. Some patients with low viral load may land up in very severe disease due to triggering of the immunological responses. Hence, again, high CT value may give a false sense of security.

I am COVID-19 positive. How do I know my CT value?

Only those who undertake the RT-PCR test can know the CT value. If your test does not explicitly state your CT value, you can ask for it.

Should I be worried if I show low CT value?

Patients in early symptomatic stage may show a high CT value which may subsequently change.

CT values differ from one kit to the other. Comparability of CT values among different kits is a challenge as our labs are using a mixed basket of kits now with different CT cut-offs and different gene targets.