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What is the value of Ct in the RT-PCR test for Corona patients?

In light of the third wave of the spread of the Corona virus around the world, it has become necessary to keep yourself updated with the terminology associated with this infectious disease from symptoms to tests. Knowing some common scientific terms related to the Corona virus can help you in early detection and prevention of the disease, and one This terminology, now part of regular use is the "Ct value", referred to in the polymerase chain reaction (RT-PCR) test, to determine whether or not a patient is positive for the coronavirus.

What is the value of Ct in the corona test?


The Ct value is a marker of viral load in a patient infected with the Coronavirus The number is reflected in the RT-PCR test report, which is considered a gold standard for detecting the presence of the Coronavirus.

The Ct value indicates the number of cycles after which the virus can be detected in the RNA collected from the patient's swab.

How is the value of Ct determined?

In the RT-PCR test, virus RNA is extracted from the swab collected from the patient's nose and throat and then the collected RNA is converted into DNA, which is subsequently amplified.

In the amplification process, multiple copies of DNA are generated to improve the chances of detecting the presence of the virus. The amplification process takes place in cycles and after multiple cycles, a detectable amount of viruses is produced.

Why is Ct important for Corona patients?

The Ct value in RT-PCR assists in determining the patient's viral load. The higher the number of cycles, the lower the viral load, and vice versa.

If the virus is detected after several cycles, this means an increase in the viral load. A low Ct value also means that there is a large amount of virus in the throat and nose of the patient and it can be transmitted to others easily, also, there is no correlation between the Ct value and the severity of the condition.

Globally, the acceptable Ct value for Corona ranges between 35 and 40 and this varies depending on the respective manufacturers' instructions for test equipment.

In India, the value of Ct is taken as 35 after considering the inputs from various virology laboratories.

A patient is considered negative if the Ct value is 35 or more on the RT-PCR test and the number below 35 is considered to be infected with the Coronavirus.

The value of Ct is considered as a parameter to determine the viral load and extent of infection in most cases of Coronavirus, however, it may have some limitations.

 The Ct value may vary depending on factors such as sample collection method, source, transportation, and time lapse in infection, collection and analysis.