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Understanding CT value in a PCR Test

CT value is the first item that we will want to see in a RT-PCR test as this value determines whether a patient is positive for Covid-19.

However, not many understands CT value and what does the number represents. To better understand CT value, we will need to first understand the test that is being conducted.

What is a serology test?

A serology test detects the presence of antibodies in the blood of an individual, including their adaptive immune response to an infection, like Covid-19. The test does not detect the virus itself, but rather the individual’s adaptive immune response.

It can show if the individual was previously infected by Covid-19 too.

Covid-19 patients most commonly will have developed antibodies around 2 weeks after they recover from the infection, and will not have them at the point of infection. The test is done by the Ministry of Health to understand if a patient has antibodies to the virus, which could imply an earlier infection.

But what is CT value?

CT value, also known as Cycle Threshold, is a value that comes up during a RT-PCR test and it is currently being labelled as the gold standard for Covid-19 testing.

In a RT-PCR test, RNA is extracted from the swab collected from the patient and then being converted into DNA. The test then looks for genetic sequences specific to Sars-CoV-2, the virus causing Covid-19.

During a PCR reaction, the genetic material from the virus is amplified and multiple copies are created. Amplification takes place through a series of cycles, a single copy becomes two, two becomes four, and so on. Only after multiple cycles is detectable quantity of virus produced.

Hence, CT value refers to the number of cycles it takes to detect the virus.

Often, a higher CT value, referring to a higher number of cycles needed, meant that the amount of virus, or viral load in the patient’s body is low. Conversely, a low number of cycles meant a high viral load.

Why is CT value important?

CT value is being used an an estimate of transmission potential.

Patients with a low CT value (high viral load) will be actively shedding the virus and is able to spread the virus to other individuals, while patients with a high CT value (low viral load) suggests that these individuals had recovered from Covid-19 and they are no longer contagious. Nonetheless, they are still actively shedding the virus but at a negligible amount.

Often, the benchmark to determine if a patient is still contagious is being set at 40, according to the Health Director-General, Tan Sri Dr Noor Hisham Abdullah.

Meanwhile, Covid-19 patients are advised to conduct a PCR test again at the end of their quarantine to ensure that they are not contagious anymore.

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Also read the statement from Dr Noor Hisham on the KKM’s interpretation of CT value here:

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