Generally, there are three types of tools for detecting new coronary pneumonia, including real-time polymerase chain reaction (RT-PCR), rapid antigen detection kit (RTK Antigen) and rapid antibody detection kit (RTK Antibody). Among them, RT-PCR is the most accurate.
The full name of the CT value is Cycle Threshold Value. The lower the CT value, the higher the virus concentration and the higher the infectivity; the higher the CT value, the lower the virus concentration and the lower the infectivity.
Because the novel Coronavirus is very small, the detection of viral RNA concentration requires amplification by PCR multiple copies of a specific gene. Each time the virus is amplified is the CT value of 1 unit, which is a power of 2.
In other words, if the viral RNA concentration is high enough to be observed by PCR detection after only a few replicates, the numerical value of the CT value will be low. On the contrary, when the PCR detection must be replicated many times to observe the viral RNA concentration, the value of the CT value is relatively high, but it also means that the infectivity is low.
About Ct value:
- According to Malaysian standards, a CT value of 40 or less is considered as a positive diagnosis of COVID-19, while a CT value of 40 or more is considered negative.
- However, the international community currently does not have a consistent standard for CT values. For example, the United States and Japan define CT values above 40 as negative, while Taiwan defines CT values above 34 as negative.
- Generally, when a confirmed person has symptoms, the CT value will be low, which means that the virus concentration in the body is high, indicating that they have just been infected.
- If the CT value is lower than 30, it usually means that it is in the early stage of infection, and it is easy to infect other people. If it is lower than 20 to 25, it means that the viral load is very high and it is easy to infect more people. If it’s below 20, it’s even possible to become a super-spreader.
- Generally, after a few days or dozens of days of infection, the viral load will become less and less, and the CT value will gradually increase
- If it is above 30, it means that the infectivity is quite low, and the patient may be in the recovery period.
- If the CT value is higher than 34 to 40, there is little infectivity.
