Explained: Why CT Value Is Important In Covid-19 Test & How to Interpret It

The ‘CT value’ indicated in RT-PCR tests basically helps in determining whether a patient is Covid-19 positive or not.

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There is little choice with us in the pandemic times, but to increase our awareness around scientific terms that can have a bearing on us. One such terminology referred to as ‘CT Value’ has become popular as the viral disease swept the country leading to deaths. The ‘Ct value’ indicated in RT-PCR tests basically helps in determining whether a patient is Covid-19 positive or not.

In fact, the Maharashtra government has recently sought clarity from the Indian Council of Medical Research (ICMR) on this subject.

As per the Indian Express report, the state wanted to understand if a person can be considered Covid-negative with the Ct value more than 24 and the person is asymptomatic. The ICMR DG responded back based on the inputs from virology laboratories across the country after which it arrives at a single Ct value cut-off. 

What is the CT value in the RT-PCR report?

At the time of an RT-PCR test, the Naso-Laryngeal swabs is taken from the suspected Covid-19 patient after which there are cycles of tests carried out on the sample. The patient is tested positive based on the cycle threshold (CT) value which is the number of cycles after which the virus can be detected.

It works as a criterion to check how soon after an RT-PCR test, the SARS-CoV-2 is detected in a sample.

Scientifically, a CT value signals the number of cycles for a sample to go through to amplify and bring up the viral DNA to a traceable level under given settings, according to the American Association for Clinical Chemistry (AACC).

Globally, the accepted Ct value cut-off for Covid-19 is between 35 and 40, depending on the instructions from respective manufacturers of the testing equipment.

What is the importance of CT-value?

 Basically, the CT count signifies the viral load present in the body.  As per the ICMR, a patient is tested Covid-positive if the Ct value is under 35. If the virus is detected after 35 cycles or prior to it, a patient is considered positive.

In case this benchmark is lowered to 24, as mentioned by Maharashtra in its letter to ICMR, along with the Ct values in the 25-34 range would not be considered as positive. Hence, the benchmark of 35 will determine that the suspected patient is Covid-19 positive than if the benchmark were 24.