Published correspondence at the Lancet addresses what doctors and scientists around the world have been concerned about: PCR Ct values and what they mean about infectivity of those who test positive as well as SARS-CoV-2 found on surfaces.
“Through a search of MEDLINE up to Oct 6, 2020, we identified six studies with original data on surface contamination with SARS-CoV-2 (RNA and infectious virus; appendix). Viral RNA was found in the surroundings of confirmed COVID-19 cases in 1·0–52·7% of samples. In five studies, infectious SARS-CoV-2 was not detected on any surface. In one study, infectious virus was detected in 9·2% of samples, which was mainly explained by a single patient with persistent cough and frequent sputum spitting during sampling (appendix). The findings are consistent with laboratory data showing for SARS-CoV-2 that Ct values of 29·3 (steel surface) or 29·5 (plastic surface) correlate with the detection of culturable virus, whereas Ct values of 32·5 (steel surface) or 32·7 (plastic surface) correlate with the detection of non-culturable virus.”https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30883-5/fulltext
The science of PCR tests by James Lyons-Weiler, PhD.