What is CT value in RT-PCR report?

Amid the concerns for an impending third wave of COVID-19, people are getting tested to detect the fatal infection. While various types of testing 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 CT Value in RT-PCR report?

Every RT-PCR report underscores an important parameter-- Cycle Threshold, commonly known as CT value. The value determines whether the tested individual is COVID positive or negative. 

Normal CT Value in RT-PCR

The Government of India has fixed the CT value at 35. If the CT value is below 35, the person is COVID positive and if above 35, the individual is COVID-negative. Globally, the CT value ranges between 35-40 to determine the infection and depends on instructions from the respective manufacturers of testing equipment.

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How is CT value determined and measured?

In the RT-PCR test, the RNA is first extracted from the swab sample, converted to DNA, and is then multiplied. The multiplication happens over a number of cycles until the virus attains a detectable level.

The CT value of an RT-PCR test is the number of cycles at which fluorescence of the PCR product is detectable over and above the background signal, according to ICMR. In simpler terms, the CT value is the number of cycles post 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 less viral load and a lower CT value means high viral load. 

Does CT value indicate the severity of the disease?

Although many studies have suggested that the clinical significance of viral load is not reliable due to little difference in the viral loads in pre-symptomatic, asymptomatic and symptomatic individuals, many still consider it as a potential value to determine severity. 

The CT value greatly depends on how and from which part of the human body the sample has been collected. A poorly collected sample may reflect inappropriate CT values and may differ between nasal and oropharyngeal specimens collected from the same individual.

The technical competence of the person performing the test, calibration of the equipment, and the analytical skills of the interpreters are other major factors that determine CT value. The temperature of transportation and the time taken from collection of the sample to receipt in the lab may also adversely impact CT values.

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