According to an analysis by The New York Times last summer, the labs that keep track of Ct numbers tend to report them at 37 to 40 — meaning they run a sample through 37 to 40 amplification cycles, if needed, before deciding whether to call it positive or negative. That could mean at least some of the people deemed positive for COVID-19 started off with minuscule amounts of virus. And while there’s no direct evidence that people with those high cycle numbers would be less sick than those with low numbers, there’s good evidence that they would at least be less contagious.
The Times analysis found, among other things, that the New York state laboratory uses a Ct of 40 to make a positive designation. If it used a Ct of 30 as a cutoff instead, the Times calculated, 63% of the people identified as positive would instead be told that no virus could be detected — in other words, they’d be told they did not have COVID-19.
Maybe I got infected months ago, and this was just the long-simmering tail of a completely hidden infection. As far as the CDC knows, tests for COVID-19 can still be positive up to three months after the symptoms resolve. So can you count three months backward, too, for someone whose symptoms never appeared? This is when it would have been really handy to know the Ct number for my test.
All my contacts tested negative for COVID-19. That was reassuring. But not completely so, since most of them used a rapid antigen test that can have disconcertingly high rates of both false-negatives and false-positives. Once again, the details of trying to chart one individual’s experience with the coronavirus revealed the patchiness of our ability to track the virus down, to test reliably for its presence, and to stay ahead of its devastation.Tested Positive For COVID Virus, But Don’t Have Symptoms. What’s That Mean? : Shots – Health News : NPR
Anything over 30 CT is about 100 percent inaccurate.