Decoding the importance of CT value in RT-PCR test

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The ongoing second wave of Covid-19 has introduced various medical terms and scientific jargons to public vocabulary which are creating speculations and unnecessary panic. One such term is ‘CT value’ in RT-PCR tests which determines whether a person is positive for COVID-19 infection. In India, two methods are followed to determine COVID infection, namely RT-PCR and Rapid Antigen Testing.

Recently, a request was sent to the Indian Council of Medical Research by Maharashtra Government to seek clarity on whether it is advisable to treat a person as COVID negative if the CT value exceeds 24 and the person is asymptomatic.

Days later, the ICMR DG issued a statement to clarify the divergent views on the CT value and stated that after taking inputs from virology laboratories across the country, a single Ct value cut-off has been determined. ICMR said that all patients with a Ct value less than 35 may be considered as positive while those with a Ct value above 35 may be considered as negative.

‘CT Value’: Understanding the Jargon

A CT value that emerges during the RT-PCR test refers to the number of cycles after which  coronavirus can be detected. According to the ICMR advisory, the Cycle Threshold or Ct value reaction is the number of cycles at which fluorescence of the PCR product is detectable. To put it in simple words, the CT value is indirectly proportional to the virus level in your body. This means, lower the CT value, the higher the virus level is in your body and consequently, the higher the CT value, the lower is the virus counts in your body because the virus has been spotted after fewer cycles.

During an RT-PCR test, RNA is extracted from the swab collected from the patient. RNA is later converted into DNA, which is then amplified. Amplification simply refers to the process of creating multiple copies of the genetic material, which in this case is, DNA. This improves the capacity of the test to detect the presence of viruses or infections.

Importance of knowing your ‘CT value’

To put the letter by the Maharashtra government in context, a patient is considered positive for COVID if the CT value in the RT-PCR test is below 35. If the value is lowered to 24, which was mentioned in the letter, then that would imply that patients with CT value ranging between 24-35 would not be considered as positive. According to ICMR, it was advisable that lowering the CT threshold parameter would only lead to missing out on several infectious people which will in turn increase transmission of disease. Globally, the accepted cut-off for CT value ranges from 35 to 40.

Decoding correlation of CT value with the severity of disease

As mentioned in the ICMR advisory, there is not enough robust evidence to support the assumption that high viral load directly does not correlate with increased infectiousness and severity of the disease. A person with a low CT value and high viral load may still be asymptomatic.

The CT value indicates the viral load in the throat as against in the lungs, which means, it is not proven that people with high CT value are safe and cannot spread the infection. Some experts suggest that the CT value does not correlate with severity, but only with infectivity. CT value, sometimes, can be influenced by various factors including temperature of transportation or the time taken from collection to receipt at the lab.