MB > "... without knowing the false positive rate..."Viral load in community SARS-CoV-2 cases varies widely and temporally:"We used laboratory, clinical and demographic evidence to classify our confidence in positive results. Around 80% had 2 or 3 genes detected (“higher” evidence), providing assurance in overall results, and all but four Ct values were under 37. Whilst Ct values are not directly comparable between studies, REACT has also validated a Ct threshold of 37 for single-gene positives for their test performed in Germany[17], and in the Public Health England (PHE) Schools study, only samples with Ct<37 were positive on repeat testing of the same swab at PHE laboratories[18]. However, every diagnostic test has false-positives, and so some of our single gene “lower”, or even “moderate”, evidence positives are inevitably false. However, the false-positive rate would be expected to be approximately constant over time, since it is either random or driven by external factors. Variation in the percentage of all tests accounted for by “lower” evidence positives, and in particular proportionate increases in “lower” evidence positives as “higher” evidence positives increased during September supports more genuinely lower-level infections occurring during the summer, and an overall false-positive rate for this test of below ∼0.005% i.e. at least 99.995% specificity." [emphasis added]Source:https://www.medrxiv.org/content/10.1101/2020.10.25.20219048v1.fullThat equates to fewer than 5 in 100,000.
John Whitworth ● 2145d