Number of Cycles in PCR tests

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Dear Public Health Scotland,

Can you please provide me with the details of how many cycles are carried out within the labs in relation to the PCR tests for Covid to determine the results (positive or negative)

Yours faithfully, Mark Paul

phs foi, Public Health Scotland

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Dear Mark Paul,

 

FREEDOM OF INFORMATION REFERENCE 2021-000630

 

Thank you for your request for information of 9^thJune 2021, received on
10^thJune 2021 requesting information about.

 

‘’Can you please provide me with the details of how many cycles are
carried out within the labs in relation to the PCR tests for Covid to
determine the results (positive or negative)’’

 

Your request is being dealt with and will be answered within 20 working
days in accordance with the Freedom of Information (Scotland) Act 2002.

 

Yours sincerely,

 

Saba ALSalahy FOI Coordinator

Public Health Scotland

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Bartholomeus Lakeman (Account suspended) left an annotation ()

It would be in the public interest to know whether the Ct value of the PCR test is the same or different for the vaccinated and the unvaccinated.
E.g., In the USA in order to encourage vaccination is the PCR test cycles threshold (Ct) for a Vaccinated person set below 28 whilst for an Unvaccinated it remains up to 40. Herewith is the test sensitivity for an unvaccinated person been set higher by >4,000 times.

phs foi, Public Health Scotland

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Dear Mark Paul,

 

Freedom of Information Reference: 2021-000630

 

Please find attached our response to your freedom of information request.

 

I hope this information is helpful to you.  Please do not hesitate to
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The FOI Reviewer

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Saba ALSalahy FOI Coordinator

Public Health Scotland

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Bartholomeus Lakeman (Account suspended) left an annotation ()

Ct (threshold cycle) is the intersection between an amplification curve and a threshold line. It is a relative measure of the concentration of target in the PCR reaction. Many factors impact the absolute value of Ct which are the concentration of the target, amount of template, artifacts from the reaction mix or instrument, environmental factors such as the pH of a solution and salt concentration, temperature, the efficiency of a PCR reaction, the performance of a real-time PCR reaction, and its Sensitivity. Of the various PCR test kits these factors are different. However, there are formulas to make their results comparable. Many labs allow a comparable Ct value of 37.

Yet there are some more troublesome biases e.g., test kit contamination, testing site or lab contamination, PCR test can react to other (6) coronaviruses, detecting non-infectious virus fragments weeks after an active infection, or from an infection of a contact person, and detecting viable virus in quantities too small to be infectious. These biases are often not accounted for.

The research group of French professor Didier Raoult has recently shown that at a Ct of 25, about 70% of samples remained positive in cell culture (i.e. were infectious); at a Ct of 30, 20% of samples remained positive; at a Ct of 35, 3% of samples remained positive; and at a Ct above 35, no sample remained positive (infectious) in cell culture. This means that if a person gets a “positive” PCR test result at a cycle threshold of 35 or higher (in the UK), the chance that the person is infectious is less than 3%. The chance that the person received a “false positive” result is 97% or higher.

In January 2021, the WHO fully confirmed the above analysis: “WHO guidance ‘Diagnostic testing for SARS-CoV-2’ states that careful interpretation of weak positive results is needed. The Ct needed to detect virus is inversely proportional to the patient’s viral load. Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different NAT technology. Some weeks after the start of the vaccination campaign the WHO advised to lower the Ct: which implies that it and the Governments knew which Ct value it was using.

Despite 18 months of PCR tests signalising Covid19 cases with its subsequence Lockdowns; currently there is no large-scale study which explores the relationship between Ct and viral load and disease severity. Only from China there are a couple of studies that explore an association between cycle threshold (Ct) values and disease severity or clinical outcome in patients infected with the SARS-CoV-2 and this with small patient sample sizes.

Several studies have since shown that national PCR testing rates have had no influence at all on covid mortality. In addition, a new German study re-analyzed PCR tests of 160,000 people and concluded “In light of our findings that more than half of individuals with positive PCR test results are unlikely to have been infectious, RT-PCR test positivity should not be taken as an accurate measure of infectious SARS-CoV-2 incidence. Our results confirm the findings of others that the routine use of ‘positive’ RT-PCR test results as the gold standard for assessing and controlling infectiousness fails to reflect the fact ‘that 50-75% of the time an individual is PCR positive, they are likely to be post-infectious.'” (Stang et al, Journal of Infection, May 2021)

Bartholomeus Lakeman (Account suspended) left an annotation ()

There are many different PCR tests used in the different laboratoriums using various Ct values: Yet to homogenise or unify the various tests results; there are formulas to get its result adhering to a certain guideline marking whether the test result is positive or negative.
In Jan- the WHO remarked that Governments allowed for a positive PCR test result a Ct value too high. This echoed that of the scientific community stating that the use of a Ct value of 27 provides about 30% false positives and a Ct of 35 about 97% false positives.
In the US, the DCD uses for testing the vaccinated a Ct of 24/28 and that for the unvaccinated up to 35 or above. For to show the 'Delta variant epidemic' the Govt uses a Ct value up to 40.
Which shows that our Govt can and does influence or select the use of the Ct value. So, when the Govt- states it holds the information then it should disclose it in a concrete way