Amid concerns about an impending third wave of COVID-19, people are getting tested for the deadly infection. While various types of tests can detect the presence of the SARS-CoV-2 virus, the RT-PCR test is considered the gold standard.

Here’s what you need to know about the CT value in the RT-PCR report.

What is the CT value in the RT-PCR report?

Each RT-PCR report highlights an important parameter: the cycle threshold, commonly referred to as the CT value. The value determines whether the person tested is COVID positive or negative.

Normal CT value in RT-PCR

The Indian government has set the CT value at 35. If the CT value is below 35, the person is COVID positive and if it is above 35, the person is COVID negative. Overall, the CT value varies between 35 and 40 to determine infection and depends on the instructions of the respective test equipment manufacturers.

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How is the CT value determined and measured?

In the RT-PCR test, RNA is first extracted from the swab sample, converted into DNA, and then multiplied. Multiplication occurs over a number of cycles until the virus reaches a detectable level.

The CT value of an RT-PCR test is the number of cycles at which the fluorescence of the PCR product is detectable above the background signal, according to the ICMR. In simpler terms, the CT value is the number of cycles after which the virus can be detected. If detected within a few cycles, the body has a high viral load and vice versa.

Thus, the CT value is inversely proportional to the amount of viral load present in the human body. A higher CT value means a lower viral load and a lower CT value means a high viral load.

Does the CT value indicate the severity of the disease?

Although many studies have suggested that the clinical significance of viral load is unreliable due to the small difference between viral loads in pre-symptomatic, asymptomatic and symptomatic individuals, many still consider it a potential value for determine the severity.

The CT value highly depends on how and from which part of the human body the sample was taken. An incorrectly collected specimen may reflect inappropriate CT values ​​and may differ between nasal and oropharyngeal specimens collected from the same individual.

The technical skill of the person performing the test, the calibration of the equipment, and the analytical skills of the interpreters are other major factors that determine the CT value. Transport temperature and the time between sample collection and receipt at the laboratory can also negatively impact CT values.

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