What is my CT value?

Many hospitals and diagnostic laboratories in Calcutta are flooded with calls asking for the value from people who tested positive for Covid-19 there, a trend that experts say is based on unfounded claims.

CT, or cycle threshold, is a number indicating the presence of a gene in a swab sample. The lower the CT value for a specific gene, the higher the presence of that gene in the sample.

Till a few days back, most people going for tests were not bothered about the CT value, said officials of several hospitals and laboratories.

But in the last three to four days, the query for the CT value has gone up for two reasons. Many are saying the doctors treating them want to know the value, while some are keen to know the number to understand whether they can infect others easily.

Microbiologists and doctors said there were no studies yet to correlate the CT value with the contagiousness of the novel coronavirus, which causes Covid-19, but still many people are demanding to know their viral load.

Officials of hospitals and laboratories feel some videos and posts on social media were triggering the interest in the CT value. In one such video, a person identifying himself as a doctor is explaining why the CT value is important.

“.... ask the lab what is my CT value. They can’t say no. If the CT value is less than 24, then it is highly contagious. Till now the cut-off was not known but studies now have shown the cut-off is 24,” the doctor says in that video.

On Monday, AMRI Hospitals Dhakuria received 15 calls from people who have tested positive for Covid-19. “They wanted to know their CT values. We are telling whoever wants to know his or her value,” an official of the hospital said.

Doctors said many people in home isolation were concerned whether they co-uld easily infect others in the family.

Experts said there was no authentic study yet to prove that a higher CT value meant a person had less chance of spreading the infection. “There is no reliable study which shows direct correlation between the CT value and the severity of the disease or contagiousness,” said Arpita Bhakta, the head of the microbiology unit at AMRI Dhakuria.

“CT values differ from one diagnostic kit to another. They also depend on how the sample is collected and in what temperature,” said Bhakta.

Pulmonologist Suranjan Mukherjee said he had no idea why some doctors were asking for the CT value. “I am not interested in the CT values of my patients who are Covid positive. Guidelines by the authorities clearly state the number of days a Covid patient needs to stay in isolation,” said Mukherjee.

Peerless Hospital chief executive Sudipta Mitra said he had seen patients with a high CT value, indicating a lower viral load, getting critical, while those with lower values not needing ICU support. “Patients in early stages of the disease are showing high CT values, but the numbers are decreasing subsequently,” he said.

According to public health experts, a high CT value can give a false sense of security.

“In our hospital, the CT value is not used by doctors for treatment. It is for academic interest,” said Abhishek Chakraborti, internal medicine resident, Brookdale Hospital Medical Centre, New York.