Just published update from French IHU group shows < 3% of C19 rtPCR “positives” at a cycle threshold (Ct) =35 yield live virus upon culture
Jaafar et al. showed that Cqs after 33 were mostly dead. Most testing facilities report a positive result if under CQ 40 (but this should be verified in your jurisdiction).
In this study 20% of samples were culture pos at ct 30 using serial blind passage. Ct values have a role to play - the challenge is determining how best to use them especially with data all over the place.
The correlation between 3790 qPCR positive samples and positive cell cultures, including 1941 SARS-CoV-2 isolates