NEW DELHI: Covid-19 patients have been flooding laboratories with requests to know the cycle threshold (CT) value of their RT-PCR test. Many believe that the CT value, which refers to the number of cycles needed in RT-PCR test to amplify the viral RNA to reach a detectable level, can help in knowing infectiousness and severity of the disease.
However, microbiologists said there weren’t enough reliable studies to show a definitive and direct correlation between the two. “RT-PCR is a qualitative and not a quantitative test where viral load can be defined clearly,” said Dr Arvind Lal, executive chairman, Dr Lal Path Labs.

RT-PCR, the gold standard for Covid-19 testing, detects the presence of infection in a clinical sample by targeting specific gene sequences of SARS-CoV2, the virus that causes Covid-19, which is usually found inside the nasal cells.
A swab sample taken from a person is put in a viral transport medium and then RNA is extracted from it. The RNA is then amplified using Polymerase Chain Reaction, which can run up to 40 cycles. However, depending on the viral load of a patient, the RNA of the virus could be detected in 15, 20 or even 30 cycles.
Dr Pratibha Kale, associate professor, clinical microbiology at Institute of Liver and Biliary Sciences, said the CT value was considered inversely proportional to the viral load. “If RNA is detectable in just 15 cycles, it will mean the viral load of the person is very high. However, there are many factors at play. For example, if the swab sample hasn’t been collected properly, the CT value would be high, but the viral load would also be high,” she added.
In many other illnesses, for example Hepatitis C, quantitative tests are available, which can tell the viral load in a patient. “In the case of Covid-19, quantitative testing is difficult because it requires tissue culture and a biosafety level-III lab, which isn’t readily available. It’s time consuming and could be dangerous given the high transmissibility of the virus,” Kale said.
Dr Navin Dang, founder and chairman of Dr Dang’s Lab, said they were flooded with requests for knowing the CT value. “We try to explain to patients that CT value doesn’t help much in knowing the infectiousness or severity of the disease, but they insist on it. Indian Council of Medical Research and Society of Clinical Microbiologists have also clarified this,” he added.
Dr Rommel Tickoo, associate director, internal medicine at Max Saket, said a high viral load didn’t necessarily lead to the infection. “Almost half the people who get Covid-19 stay healthy even after having a similar amount of the virus compared with patients who fall ill. As a physician, CT value is not the only thing I will use to decide the course of treatment,” he added.

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