CT value in Covid-19 test: what is it and why is it being discussed?

RT-PCR tests can show whether or not a person has contracted coronavirus, however, it does not reveal the amount of virus (viral load in scientific parlance) present in the patient's body. There is, however, an indirect way to detect the viral load that a person might be carrying.

This is why many doctors have been telling their patients lately that the result of their testing for Covid-19 should mention CT value, not only if the sample tested was positive or negative, as they believe there is a relation between the CT values and the severity of the symptoms.

What is the CT value?

The Ct or threshold cycle value is the cycle number at which the fluorescence generated within a reaction crosses the fluorescence threshold, a fluorescent signal significantly above the background fluorescence. At the threshold cycle, a detectable amount of amplicon product has been generated during the early exponential phase of the reaction.

According to some doctors, a Ct value of 20 or below indicates high viral load in the patient's body, in addition, Ct values enable doctors to know the severity of symptoms the patient may develop as well as help them make their decision on whether the patient can remain isolated at home or should be admitted to a hospital.

On the other hand, some experts and professors believe that there is no tangible proof of the correlation between Ct values and the severity of the disease. For instance, an article published on "Thehindu" website, where Dr. Gagandeep Kang, Professor of Microbiology at CMC Vellore, stated “In my view, there is a relationship between the cycle threshold value and the probability of shedding infectious virus, the lower the number the higher the likelihood. But there is no real evidence-based established number where you can say, above 30 [Ct value], you will definitely not be infectious. That said, by [Ct value] 35 you will unlikely be infected,” he added.