The World Health Organisation released a document entitled WHO Information Notice for IVD Users dated 14 December 2020 which, interestingly enough, seems to have disappeared from their web site "This page cannot be found" but was archived here.
How many PCR test cycles should be standard if we want 80% of the positives to represent an inoculable sample i.e. virus can actually be cultured from the sample provided by the patient? At a of Ct25 80% of the samples termed positive by the RT-PCR test will be inoculable. However, 20% of the positives will be false, if inoculation is a bench-mark for efficacy of the RT-PCR"