I believe the "positive" range that is in consideration now is anything up to a Ct of 33. 21 is absurdly low, especially for RNA fairly crudely extracted from saliva/phlegm. A difference in Ct value of 1 is equivalent to a difference in expression of 10^1, and so on. So considering a Ct value of 21 to be the upper threshold rather than 30-33 is saying the body must have 10^9 - 10^12 additional copies of viral RNA present, which is a massive difference.
And sorry I don't have a source on that, I just do a lot of PCR.
I believe the "positive" range that is in consideration now is anything up to a Ct of 33. 21 is absurdly low, especially for RNA fairly crudely extracted from saliva/phlegm. A difference in Ct value of 1 is equivalent to a difference in expression of 10^1, and so on. So considering a Ct value of 21 to be the upper threshold rather than 30-33 is saying the body must have 10^9 - 10^12 additional copies of viral RNA present, which is a massive difference.
And sorry I don't have a source on that, I just do a lot of PCR.