SAGE Exclusive: PCR Tests With a Ct >25 Pick Up Non-Infectious Positives. No, really?

 Wed 11:56 am +01:00, 10 Feb 2021  1

posted by Weaver

The SAGE minutes from December 21st contain a striking admission: that PCR tests above the 25 cycle threshold (Ct) are “not associated with transmission-type patterns meaning these people may be less infectious to others than those whose tests have a low Ct value”. This, they say, is a key reason why Lateral Flow Tests, though less sensitive, are no less reliable than PCR for containing the outbreak: “Lateral flow testing (which is more likely to identify cases which have a low PCR test Ct value) is better at identifying more infectious individuals than it is at identifying infected but less infectious people”. Here is the relevant section:

Use of household survey to measure infectiousness

39. Cycle threshold (Ct) values broadly categorise the concentration of viral genetic material in a patient sample following testing by RT-PCR (with higher Ct values corresponding to lower concentrations). Initial analysis from ONS COVID-19 Infection Survey (CIS) data up to December 7th showed that positive tests with high Ct values (>25) do not cluster with other positive tests (with either high or low Ct values).

40. This suggests that they are not associated with transmission-type patterns meaning these people may be less infectious to others than those whose tests have a low Ct value. Some caution is required with this finding, as higher Ct values were observed during the summer when other factors may have reduced within household transmission. However, it would be in line with expert opinion which suggests a Ct value of below 25 seems to be associated with viable transmission.

41. This also supports the hypothesis that lateral flow testing (which is more likely to identify cases which have a low PCR test Ct value) is better at identifying more infectious individuals than it is at identifying infected but less infectious people.

This is a vindication of an argument first advanced by sceptics like Dr Clare Craig, who saw the advantages of LFT over PCR tests in terms of avoiding false (non-infectious) positives back in November.

https://lockdownsceptics.org/