Every COVID-19 test report has a Ct value. The cut off of Ct value determines whether the person should be declared COVID positive or not.
Last year, Maharashtra state government had requested clarity from the Centre on the cut off of Ct value and had asked whether an asymptomatic person with Ct value more than 24 can be treated as COVID negative. This, the state government had said in the letter, is in view of the divergent views and Ct values shared by the ICMR, NITI Aayog and NCDC.
To this the central government had replied that it has decided to fix a single cut-off for Ct value following consultation and inputs from virology laboratories across the country.
The government has set the Ct value at 35 to determine whether a person is COVID positive or not.
A Ct value below 35 suggests the tested individual is COVID positive and should take medication and self-isolate quickly to stop spreading the coronavirus further.
A Ct value above 35 suggests the tested individual is COVID negative and can self-isolate only if there are other COVID positive individuals in the vicinity.
Worldwide, the Ct value is set within a range of 35-40 to determine COVID infection.
So how does Ct value decide if you have the infection or not?
Ct value stands for cycle threshold. During the RT-PCR test, the Ct value determines the detectable level of the virus in a given sample based on which it is decided if the person is COVID positive or negative.
In RT PCR, the RNA is extracted from the sample and is converted to DNA, which is then amplified or multiplied. This multiplication happens over a number of cycles in order to find out the viral load.
If the virus is found within fewer cycles, this would mean that the body has a high viral load. If the virus is found after more cycles, this means the body has lesser viruses which could only be detected after the sample was passed through many cycles.
In this way, the Ct value is inversely proportional to the amount of viral load in the human body. Higher the Ct value lesser the viral load and lower the Ct value, higher the viral load.
Does Ct value determine the severity of the infection?
This still remains a question.
Studies have revealed that the clinical significance of viral load is not reliable as there is little difference in the viral loads in pre-symptomatic, asymptomatic and symptomatic patients.
However many researchers consider Ct value as a potential parameter to determine severity.
Monica Gandhi, an infectious diseases specialist at the University of California, San Francisco says, “It’s fair to say that having a higher viral load is associated with being more infectious,” based on her research study on 3790 samples.
Gandhi says that access to Ct values could be helpful but at the same time she also says that a high viral load doesn’t necessarily lead to disease as close to 40% people stay healthy even after contracting the disease.
Pressing further on the importance of Ct value, in a report carried by Science, Michael Mina suggests contact tracing, which is so far considered to be a key strategy for interrupting the chain of transmission, should be done in the order of Ct values. Contact tracing for those with high viral loads should be put before those with low viral load or higher Ct value, according to him. “We have to stop thinking of people as positive or negative, and ask how positive?” says the physician and epidemiologist at Harvard University’s T.H. Chan School of Public Health, stressing on the importance of Ct value in determining the infection level in individual.
While whether Ct value alone can determine the risks, severity and infection level in an individual still remains debatable, many clinicians, researchers consider it as one of the many parameters that hold potential in determining the dynamics of the infection.
“As a physician, having the CT value is not the only thing I will use,” says Chanu Rhee, a hospital epidemiologist at Brigham and Women’s Hospital. But I do find it helpful, Rhee adds.
Likewise, in a study on the correlation between the viral load or Ct value and progression of COVID-19, a team of researchers in China had found that viral load was negatively correlated with lymphocyte count but positively correlated with neutrophil count and C-reactive protein; which associates with inflammation, a key feature of COVID-19 infection. Citing limitations in their study, they have said that though in some patients viral load tended to increase and Ct value decreased during the conversion from mild cases to severe cases, it was not the only reason for the deterioration of the disease.
Why this contention?
“Unfortunately, the use of raw Ct data, inappropriate references for normalization, or even non-standardization are being widely considered, ” say researchers in the “Misinterpretation of viral load in COVID-19 clinical outcomes” paper. “Knowledge of viral load is essential to formulate strategies for antiviral treatment, vaccination, and epidemiological control of COVID-19. Moreover, identification of patients with high viral loads can also be useful to understand risk factors such as age, comorbidities, severity of symptoms and hypoxia, to decide on the need for hospitalization,” they say and have added that, “However, in a great number of emerging studies, cycle threshold (Ct) values alone are often used as viral load indicators, which may be a mistake.”
Diversity of sample types, quantities of starting material for RT-qPCR, differences among commercial detection kits, experimental conditions, and real-time equipment for COVID-19 diagnosis are attributable to misinterpretation of Ct values, researchers say.
Ct values may also differ in the same person if the sample is taken from different parts. Ct values between nasal and oropharyngeal specimens collected from the same individual may differ. “Ct values also depend on how the sample has been collected. A poorly collected sample may reflect inappropriate Ct values. Besides, Ct values are also determined by technical competence of the person performing the test, calibration of equipment and pipettes and analytical skills of the interpreters,” the government says.
Ct value in India
In a 2020 advisory, the government has said, “It is being assumed by some researchers / clinicians that high viral load directly correlates with increased infectiousness and severity of disease. However, the evidence is not robust enough to definitively support this assumption. Recent discussions about guiding the clinical decision-making process based on the Ct values of the RT-PCR test reported by laboratories has several limitations.”
“Severity of COVID-19 disease largely depends on host factors besides the viral load. Some patients with low viral load may land up in very severe disease due to triggering of the immunological responses,” the government has advised people.
“Hence, again high Ct value may give a false sense of security,” it has said pressing on the need for following COVID prevention guidelines.
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