What is the CT value in the Covid-19 PCR test?

Polymerase Chain Reaction (PCR) Tests are not at all a new name for people these days. It is something that is constantly in the news, media, and discussions of people. 

PCR Tests are currently widely used to test if a person has the SARS-CoV-2. It is the most reliable method to test for the COVID-19 virus for now. 

However, now the test centers not only report if the PCR Test result was positive or negative. But should also include a number known as the Cycle Threshold (CT). So what is this number and what does it signify?

PCR Test: How does it work?

We need to first understand the working of the PCR Test. There are three key steps to the process.

Sample collection

The collection of samples is through a swab of the nasal cavity. These swabs are sealed in a tube and sent to the laboratory.


This is a process that isolates genetic material from any virus that may be present from the swab, and it is sent to further testing.


This step includes a thermal cycler and chemicals which are used to make a reaction. This reaction makes millions of copies of a small portion of the virus’s genetic material. During this process, one of the chemicals produces a fluorescent light if the virus is present in the sample. This light is detected by the PCR machine and special software is used to interpret the signal as positive or negative. 

Also read: Covid19 Emergencies: All you need to know.

Cycle Threshold 

The CT value is the number of cycles necessary to spot the virus. The PCR machines stop running at that point. If a positive signal isn’t seen after 37-40 cycles the test is negative. The lower the CT value the higher the quantity of viral genetic material in the sample.

Hence, this number is the value that indicates how much virus is harbored in the infected person. Advocates point out that if this value is also given with the PCR Result, it will help understand for doctors as well as the patient itself the severity of their case.

This means doctors can flag patients at a higher risk and sort out priorities accordingly.

Why not release it?

However, the reason this value was not given out with the result is that they are slightly unreliable. The same sample can give different CT values on different testing machines and different swabs of the same person. 

This does not make it trustworthy, which is why it was not given out. According to TheHindu.com, there are three reasons why people think sharing CT value is not a good idea.

Lack of studies

There are no studies to show the correlation of disease severity and CT value. The CT value may be correlated more towards the person than the virus. There is simply no real evidence-based quantity that can guarantee safety.

Relative measure 

Unlike samples of blood, the PCR tests are not an absolute measure but only a relative measure of the virus load. This means the CT is also only a rough estimate. The CT values can hence differ depending on the method of sample collection and also depends on the lab technician. This is why some people can test positive one day and negative the other.

False security

Patients in the early stages may have a less viral load, which means a high CT value but this can change as the days pass by creating false security of patients. Besides the virus load, the severity of the disease also depends upon the host factors such as immunity. Some may suffer more or less on the same amount of value.

Nevertheless, people argue that knowing the CT values can be highly informative because hospitals that are packed could discharge and free up space for the ones who are more at risk. 

Do you think these CT values should be released with the test results? Will knowing the CT value ensure proper management and more direct care or will the unreliability of the CT value instead cause a great mishap and spread the virus further?

Share your thoughts.

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