What new Covid-19 jargon 'CT values' means and why they're important

Source: 1News

New Zealanders are no strangers to snap lockdowns, but we are to some of the lingo. This latest lift in alert levels has brought with it a new piece of Covid jardon: "CT values".

During Sunday's alert level announcement, Covid-19 Response Minister Chris Hipkins talked about the CT values of those who'd tested positive for the virus.

For many people it was the first time they'd heard of CT values, so what are they exactly and why are they important?

"[The recent cases are] positive tests with relatively low CT values which indicates they are new and active infections," Hipkins said.

A CT - cycle threshold - value tells you how much of the viral genome is present in the sample, microbiologist and immunologist James Ussher, an associate professor at the University of Otago, told Seven Sharp.

A cycle threshold value is a figure indicating how much Covid-19 a person has in their body. 

When you take a test, scientists copy the sample again and again until they find what they're looking for. 

The CT value is the number of copies it takes for the virus to show up.

A low rate of infection will take more cycles to show up but if the patient is highly infectious, it'll show up quickly, so your CT value would be low.

"The more virus there is or the more target there is, the earlier the CT or the cycle threshold becomes positive. The less there is, the later it is - it's the inverse," Ussher said.

"It's part of the puzzle that can be used to decide when infection may have occurred."

The jury's still out on whether health authorities should announce the CT value with positive cases of the virus.

"I think it's an important part of the puzzle for those ... trying to put infection in context or the result in context," Ussher says.

"I don't think it's necessarily helpful for the public to know. What people want to know, is it a recent infection or whether it is a historic infection and whether the person is infectious or not, and the CT value isn't able to tell us any of those things on its own."