CT score in COVID-19 result: What it means in RT-PCR report & what is the normal score?

The COVID-19 virus testing has one question in common for every suspected patient which is the cycle threshold (CT) value or CT score which basically decides whether a patient is positive for the virus. The medical experts have directed that a patient's result should report not just whether a person is positive, but also a number of CT values, which indicates how much virus an infected person carries. Studies have also pointed that the ct score could help doctors flag patients at high risk for serious disease.

This was also the matter of a recent request sent by the Maharashtra government to the Indian Council of Medical Research (ICMR). The state sought clarity on whether it was advisable to treat a person as COVID-negative if the Ct value is more than 24 and the person is asymptomatic.

What is CT score in COVID report ?

According to the State health officials, various ICMR documents had mentioned different Ct values and there were divergent views even among Niti Aayog and the National Centre for Disease Control. Ct is a figure that emerges during RT-PCR tests, the gold standard for detection of the SARS-CoV-2 coronavirus. In an RT-PCR test, RNA is yanked from the swab collected from the patient. It is then converted into DNA, which is then intensified. Amplification refers to the process of creating multiple copies of the genetic material — in this case, DNA. The process helps to detect the presence of the virus in an accurate way. Amplification takes place after multiple cycles that a detectable amount of virus is produced.

What is normal CT score in COVID?

It is suggested that a higher CT value indicates a low viral load. However, health experts have also mentioned that some patients can have a high CT value and yet have a very significant level of COVID-19 infection, and vice versa. Many factors are important in diagnosing an RT-PCR test, and the results may also depend on the method of specimen collection and time from infection to collection and to analysis. 

An August 2021 ICMR advisory highlighted that Ct values depend on how the sample has been collected. An inadequately collected sample may reflect inappropriate Ct values. Besides, Ct values are also determined by the technical competence of the person performing the test, calibration of the equipment, and the analytical skills of the interpreters.