What is Ct Value in RT-PCR Test and Why is it Important?


As the COVID-19 cases are surging in the country and several post-COVID complications are coming into limelight, keeping up with the unique terms has become very important. One should know some common scientific terms linked with the COVID-19 for timely detection and medication of the infection. ‘Ct value,’ a common term for people who have got themselves tested for COVID-19. This value is reflected in the reverse transcription-polymerase chain reaction (RT-PCR) test, which has been identified as the gold standard for detecting COVID infection in people.

What do you mean by Ct value?

Ct value identifies the intensity of the infection in a patient with SARS-CoV-2 virus. The Ct value number is reflected in the RT-PCR report which is considered as the most trustable method to detect infection within people. To boil it down in simple words, Ct value is the number of cycles after which the virus is detected in the RNA collection of a patient. The RNA is collected through swabs from nose and throat, further RT-PCR test takes around 24 to 48 hours to give out results.

How is Ct value obtained?

RNA collected from swabs is first converted into DNA, which later gets amplified. The amplification process includes creation of multiple DNA copies, which improves the possibility to detect the presence of virus. Amplification does not end after one cycle; several cycles take place before the detectable amount of virus is produced. Ct value here reflects the number of cycles at which the virus becomes detectable.

Importance of Ct value?

As informed above the Ct value concludes the intensity of the virus in a patient. If the virus is detected after a few cycles, then the viral load of the patient is much higher. Ct value and the viral load are indirectly proportional. If Ct value rises, then the viral load is lower and vice versa. As the swabs are collected from throat and nose, a lower Ct value makes the person capable of transmitting the disease to other people. Therefore, people infected with the virus are suggested to remain isolated till their Ct value does not cross the threshold. But here a point is to be remembered that the Ct value does not reflect the severity of the case.

Threshold of Ct value?

Doctors and medical experts have given the Ct value cut off for COVID-19 virus between 35 and 40. A patient is said to be negative if his/her Ct value comes out to be 35 or more in the RT-PCR test and positive if the number falls below 35.

However, there are some limitations to the Ct value. Several factors including transportation, method of specimen collection, and time gap in contracting the infection, collecting, and analysis may affect the Ct value reflected in the RT-PCR test.

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