CT value in RT PCR result, How much is dangerous?

Ground Report | New Delhi: CT value in RT PCR result; India is recovering from a third explosive wave of the Covid outbreak led by the Omicron variant as the country reports over three lakh cases. Among several scientific terms that the Covid-19 pandemic has made part of the public vocabulary, one is the 'Ct value' in RT-PCR tests to determine if a patient is positive for Covid-19.

CT value in RT PCR result covid

We are currently in the phase of the third wave of COVID-19. COVID positive cases are rapidly rising. Though some of us are going to test centres for confirmation, many others are getting tested at home. In all the testing procedures the presence of the infection is being determined by the Ct value.

In every report of COVID-19, there is a Ct value.  The Ct value numeric determines whether the person should be declared COVID positive or not.

Earlier this was an important subject sent by the Maharashtra government to the Indian council of medical research to know covid negative whether it is advisable to treat a person having a CT value of more than 25 and the person is asymptomatic.

In the laboratory, the DNA strand goes through several cycles of PCR until it replicates. The CT value refers to the number of cycles it takes for the DNA to reach a measurable level. Supposedly, if the DNA undergoes 15 cycles of PCR before the dye is detected, then its CT value is 15.

After various inputs are taken from virology laboratories from all over the country the Indian Council of Medical Research( ICMR) have decided on a CT value cut-off. Patients having less than 35 are now considered as positive and those who are having a ct value above 35 they are considered as negative.

What is ct value is and why it is important?

So basically ct is known as Cycle Threshold, ct is a numeric value that appears in RT-PCR test reports and to detect the SARS cov-2. CT value is the cycles at which fluorescence of the PCR is detectable over and above the background.

The ct value decides whether the person is infected with COVID or not. For example, if the Ct value is low it means the virus was detected in lesser cycles which means that the body has a high chance of virus present in the body.

According to the ICMR, a patient is considered covid positive if the Ct value is below 35. In other words, if the virus is detectable after 35 cycles or earlier, then the patient is considered positive. If the reference point were lowered to 24, the value mentioned in the letter from Maharashtra, it would mean that Ct values in the range of 25 to 34 would not be considered positive. Therefore, a reference point of 35 means that more patients would be considered positive than we would if the reference point were 24. The ICMR has said that lowering the Ct threshold parameter may lead to missing several infectious people.

Following are consolidated recommendations of ICMR “Expert Group”

As per research, the CT value not only gives information about the presence of the coronavirus, it also aids in understanding the behaviour of the virus in due course of infection. Though, it has clinical restrictions owing to various factors of which the important ones are technical competence of the person performing the test,  gauge of equipment and analytical skills of the interpreters.

• There are no reliable studies to conclusively prove a direct relationship between disease severity/infectivity and Ct values. Viral load does not play much of a role in patient management.

• Ct values ​​vary from one kit to another. Comparison of Ct values ​​between different kits is a challenge as our laboratories are now using a mixed basket of kits with different Ct cut-offs and different gene targets.

• Ct values ​​also depend on how the sample has been collected. A poorly collected sample may reflect inappropriate Ct values. In addition, CT values ​​are also determined by the technical ability of the person performing the test, the calibration of equipment and pipettes, and the analytical skills of the interpreters.

• Ct values ​​may differ between nasal and oropharyngeal specimens collected from the same individual. Similarly, the temperature of transport, as well as the time taken from collection to receipt in the laboratory, can also adversely affect Ct values.

• Samples from asymptomatic/mild cases show Ct values ​​similar to those who develop severe disease. - Patients in the early symptomatic stage may show high Ct values ​​which may change later. In such cases, high Ct values ​​will give a false sense of security.

• The severity of the COVID-19 disease largely depends on host factors other than the viral load. Some patients with low viral loads may develop very severe diseases due to the triggering of immunological responses. Therefore, again a high Ct value can give a false sense of security.

• Moreover, the RT-PCR test presently being conducted is qualitative in nature. Ct values may give a rough estimate of viral load. However, more specialized standards are required for quantitative assays which are currently unavailable for SARS-CoV-2.

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