COVID-19: CT value can tell your viral load, but it has many limitations

Mumbai-based diagnostic laboratory, iGenetic Diagnostics, began providing Cycle Threshold (CT) value on Reverse Transcription–Polymerase Chain Reaction (RT-PCR) test reports of patients who tested positive for COVID-19.

Real time RT–PCR is a nuclear-derived method for detecting the presence of specific genetic material in any pathogen, including a virus.

“CT value may not be clinically significant, but we are providing it following demand from doctors and patients,” says Arunima Patel, Founder and MD, iGenetic Diagnostics.

Patel says that they give a disclaimer in the test report, clearly mentioning the limits of the CT value.

iGenetic isn’t alone. Most diagnostic lab chains are providing CT value on their RT-PCR test report for patients who have tested COVID-19 positive. To be sure, ICMR doesn’t mandate diagnostic labs to provide CT value.

A PCR test works by amplifying the genetic material of Coronavirus through multiple cycles. The number of cycles needed to detect the virus is called the cycle threshold or CT.

If the virus amount or viral load is high, the CT value would be lower. If the viral load is lower, the CT value would be higher. Studies have found that it would be difficult to culture the virus, if the CT value is greater than 34, implying that the person may not be infectious.

The CT value becomes important as it helps to identify and isolate infectious people. People with high viral load are quarantined, while those with insignificant viral load could be asked to get themselves quarantined at home.

Currently, RT-PCR only tells us whether a person is positive or negative. This information will help get both qualitative and quantitative information.

Karnataka and Odisha have written to ICMR to provide CT values in the test reports.

While the CT value helps understand the viral load of the person, it has several limitations.

“Some researchers and clinicians assume 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,” said ICMR in its recommendation.

In addition to lack of evidence supporting correlation, ICMR points to factors such as differing CT values among different test kits, sample collection procedures, temperature during transportation and time taken from the collection centre to the lab impacting CT values.

ICMR also says that patients in early symptomatic stage may show a high CT value or low viral load, which may subsequently change. In such cases, it says high CT values will give a false sense of security.

Doctors like Farah Ingale, who is heading the COVID ward at Fortis, Vashi, concurs with the ICMR’s recommendation.

“Initially, people were talking about reporting CT value to quarantine people with high viral load,” Ingale said.

“The problem with CT value is, it is not reliable. It varies from test kit to test kit. If you do the CT cycle threshold of a same patient twice, it throws up two different numbers,” she added.