What is the Ct value?
Ct value, short of cycle threshold, is the marker of viral load in a patient infected with SARS-CoV-2. The number is reflected in the RT-PCR test report, which is considered a gold standard for detection of the presence of coronavirus. In simple words, the Ct value refers to the number of cycles after which the virus can be detected in the RNA collected from the swab of the patient.
How is the Ct value determined?
In the RT-PCR test, the RNA of the virus is extracted from the swab collected from the nose and throat of the patient. The collected RNA is then converted into DNA, which is later amplified. In the Amplification process, multiple copies of the DNA are created to improve the chances of detecting the presence of the virus. The process of amplification takes place in cycles and after multiple cycles, a detectable amount of virus is produced. Ct Value is referred to the number of cycles at which the fluorescent signal of the PCR product crosses the threshold and becomes detectable.
Why is Ct value important?
The Ct value in the RT-PCR test helps to determine the viral load in the patient. The higher the number of cycles, the lower the viral load and vice versa. If the virus is spotted after a few cycles that means higher viral load. Low Ct value also means that a high amount of virus is present in the throat and nose of the patient that can be transmitted to others easily. Also, there is no correlation between the Ct value and the severity of the case.
The significance of the threshold of 35?
Globally, the acceptable Ct value cut off for COVID-19 is between 35 and 40. This differs depending on the instructions from respective manufacturers of the testing equipment. In India, the IMCR has indicated taking the Ct value as 35 after considering the inputs from various virology labs.
A patient is considered COVID-19 negative if the Ct value is 35 or more in the RT-PCR test. The number below 35 is considered COVID-19 positive.
The Ct value is considered as the parameter to determine the viral load and contagiousness in most coronavirus cases, however, it might have some limitations. The Ct value may vary depending on the factors like the method of specimen collection, source, transportation, and the time gap in contracting the infection, collection, and analysis.