What does Ct value in Covid-19 test means? ICMR explains

May 3: Ct value or ‘Cycle Threshold’ value which emerges during the RT-PCR tests, the gold standard for detection of SARS-COV-2. According to Indian Council of Medical Research (ICMR), Ct value is inversely proportional to the amount of generic material (RNA) in the starting sample and lower Ct values generally correlate with high viral load. (Source: Inshorts & ICMR)

A person is Covid positive if Ct value is below 35, that is, if the virus is detectable after 35 cycles or earlier. Ct value can also be referred via a chest scan for patients who may get a negative RT-PCR/RAT report due to insufficient viral load detection in the nose and throat samples.

Some researchers suggest that high viral load directly correlates with increased severity of disease. However, ICMR states that there is no reliable studies to definitely prove direct correlation between disease severity and Ct values. Viral load does not have much role to play in patient management.

So do people with severity of disease always report a low Ct value? According to ICMR, asymptomatic/ mild cases can show Ct values similar to those who develop severe disease. Patients in early symptomatic stage may even show high Ct values that can give a false sense of security. Severity of disease largely depends on host factors besides viral load.

Now, how far accurate is the Ct value always? A poorly collected sample may reflect inappropriate Ct values. Temperature of transportation and time taken from collection to receipt can also adversely impact Ct values. Besides, Ct values are determined by technical competence of the person performing tests, calibration of equipment, pipettes and analytical skills of interpreters.