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Should guidelines be evidence-based?
The PCR test works by duplicating the genetic material in a sample to a point where a specific genetic sequence is detected.
Ct is the cycle threshold. It is the number of amplification cycles needed ...
1/n
The PCR test works by duplicating the genetic material in a sample to a point where a specific genetic sequence is detected.
Ct is the cycle threshold. It is the number of amplification cycles needed ...
1/n
..to see the fluorescent color which indicates the presence of the genetic sequence.
The higher the Ct it is set on, the more likely it is to detect a lower viral load. 2/n
The higher the Ct it is set on, the more likely it is to detect a lower viral load. 2/n
So if a person has recovered and has a minute amount of viral debris still in his body:
by setting the Ct on 30, the viral debris won't be detected => giving a (-) test result
by setting the Ct on 40, the viral debris will be detected
=> giving a False (+) test result 3/n
by setting the Ct on 30, the viral debris won't be detected => giving a (-) test result
by setting the Ct on 40, the viral debris will be detected
=> giving a False (+) test result 3/n
The WHO's guidelines: Ct=47
https://www.who.int/diagnostics_laboratory/eul_0489_185_00_path_covid19_ce_ivd_ifu_issue_2.0.pdf?ua=1
4/n
https://www.who.int/diagnostics_laboratory/eul_0489_185_00_path_covid19_ce_ivd_ifu_issue_2.0.pdf?ua=1
4/n
The NHS England Guidelines: Ct= 46 cycles
https://www.england.nhs.uk/coronavirus/wp-content/uploads/sites/52/2020/03/guidance-and-sop-covid-19-virus-testing-in-nhs-laboratories-v1.pdf 6/n
https://www.england.nhs.uk/coronavirus/wp-content/uploads/sites/52/2020/03/guidance-and-sop-covid-19-virus-testing-in-nhs-laboratories-v1.pdf 6/n
"viral culture & isolation is regarded as a gold standard or reference test against which any diagnostic index test for viruses must be measured and calibrated" 7/n
https://www.cebm.net/covid-19/infectious-positive-pcr-test-result-covid-19/
https://www.cebm.net/covid-19/infectious-positive-pcr-test-result-covid-19/
So they use viral culture to figure out at which Ct cute-off, you are likely to get a live virus or a dead one.
Here's what @carlheneghan (CEBM- Oxford) found:
âA cut-off RT-PCR Ct > 30 was associated with non-infectious samples.â 8/n https://www.medrxiv.org/content/10.1101/2020.08.04.20167932v3
Here's what @carlheneghan (CEBM- Oxford) found:
âA cut-off RT-PCR Ct > 30 was associated with non-infectious samples.â 8/n https://www.medrxiv.org/content/10.1101/2020.08.04.20167932v3
Here's another study:
âThe lowest viral load (Ct value) for which there was positive culture growth was 34.3.â
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7323671/
No live viruses are found at CT values of 30 or 34 and above.
All what these tests are detecting is old debris of the virus. 9/n
âThe lowest viral load (Ct value) for which there was positive culture growth was 34.3.â
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7323671/
No live viruses are found at CT values of 30 or 34 and above.
All what these tests are detecting is old debris of the virus. 9/n
Is it time to listen to the evidence and change the guidelines?
Reducing the Ct cut-off of the PCR test would reduce the false positives, the #Casedemic and the unjustified restrictions on people's lives. 10/10
Reducing the Ct cut-off of the PCR test would reduce the false positives, the #Casedemic and the unjustified restrictions on people's lives. 10/10
On 8/n
Typo: Cut-off
Typo: Cut-off
Prof Carl Heneghan recommends a Ct cutoff of 30. https://twitter.com/carlheneghan/status/1303220734689501184
âSARS-CoV-2 cell infectivity was only observed for RT-PCR Ct < 24 and STT < 8 days.â
STT: Symptom To Test
To diagnose:
1) PCR (+/-)
2) Ct (cycle threshold)
3)Time of symptom onset (to determine time point of infection) https://pubmed.ncbi.nlm.nih.gov/32442256/
STT: Symptom To Test
To diagnose:
1) PCR (+/-)
2) Ct (cycle threshold)
3)Time of symptom onset (to determine time point of infection) https://pubmed.ncbi.nlm.nih.gov/32442256/
