Can someone explain why a CT value of 40 is used. Surely way to high. I have read that a value of around 30 is more reasonable for this ropey test.
Because its nowhere near as simple as that.
There is no magic Ct where below which its a non issue.
They've managed to culture virus from Cts of 38+ but also failed on some samples of 20. Loads of viable cultures at 35 or so.
Its a distribution curve of probabilities.
Using 40 or whatever isn't specifically a problem. The problem relies on using a simple yes/no as a diagnostic.
They should be reporting the Ct and combining that with symptoms or close contact data to determine infection probability. As far as im aware this is only done in some hospital settings, not at lighthouse labs.
The problem isnt the test - its how the results from it are used.
Many Thanks. I know the test can't determine between live or dead virus. But what would generally be recognised as the confidence level for detecting "dead or alive" viral RNA, or is there a fear of missing "positive" cases by trying to detect a low viral load.
If you're worried purely about disease prevention then a false negative is more dangerous than a false positive.
Standard public health there.
Ultimately you cant just set a Ct (also each different assay has a different threshold so its not just viral rna related) and use it.
It has to be combined with other factors such as symptoms and/or likely exposure to someone infected. The problem we have is its not. The test is the only diagnostic.
No its not. There is no "right" Ct.
There is no number you can pick and say "if its below this its probably fine".
It doesnt work like that and cant be used like that.
If you can culture virus from a particular same at a Ct of 38 then 38 is not too high *in that situation*.
PCR is supposed to be a confirmatory diagnostic in conjunction with a clinical diagnoses. Its not designed to be a stand alone test.