Public Accountability Unit
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Our ref: 08/03/cf/2985
17 March 2021
Dear Vicky Stewart, Re: Case ref: 2985 - Freedom of Information - PCR amplification cycle number
Thank you for your request received on 08 March 2021 addressed to Public Health
England (PHE). In accordance with Section 1(1)(a) of the Freedom of Information
Act 2000 (the Act), I can confirm that PHE does not hold the information as specified
by your request.
Regarding the use of the PCR test to test DNA samples for the viral RNA of the
How many amplification cycles are being used when searching for different
hallmarks of the SARS-CoV-2 virus as standard?
Please can you provide me with the number of amplification cycles used as
standard for the aforementioned test on a weekly basis, from 1st March 2020
up until today the 8th March 2021 when testing UK citizens for SARS-CoV-2.
If Public Health England is not the authority issuing guidance for the number
of amplification cycles used, please direct me to the appropriate authority
which is in charge of making this decision.
Under Section 16 of the Freedom of Information (FOI) Act, public authorities have a
duty to provide advice and assistance, please see the below information and
guidance that explain PCR testing and cycle threshold.
There over 80 different commercial PCR tests and several platforms used in the UK
– each comes with its own standardised interpretation – some may provide Ct values
some may not. All labs are asked to perform preliminary checks on the limits of
detection and create a standard operating procedure based on guidance above to
interpret a positive or negative result.
The below document provides guidance for procedures to be implemented in
laboratories to provide assurance of positive SARS-CoV-2 RNA results during
periods of low prevalence, when there is a reduction in the predictive value of
positive test results.
This guidance is applicable to all laboratories to help guide the interpretation of
positive results. The document provides background information, explains the
problem and details actions that should be considered by all laboratories providing
SARS-CoV-2 RNA testing. https://www.gov.uk/government/publications/sars-cov-2-rna-testing-assurance-of-
There is an extensive menu of tests currently used to detect SARS-CoV-2 in a
sample. Whilst most of these tests are PCR based, there are also assays that use
alternative technology for the detection of SARS-CoV-2 nucleic acid.
Currently there are over 80 different testing platforms in use across the various
networks of laboratories and the thresholds for result interpretation will be configured
in each case by the manufacturers of the assays and the instrumentation.
Each laboratory will regularly assess the performance of their assays within their
local quality management system. The results are monitored in accordance with
defined quality control procedures. Whilst each laboratory has a statutory duty to
report positive cases to Public Health England, additional information such as Ct
values in the case of PCR tests, is not submitted.
Understanding cycle threshold (Ct) in SARS-CoV-2 RT-PCR – may be a document
that you find useful. Please see the link below: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attach
If you have any queries regarding the information that has been supplied to you,
please refer your query to me in writing in the first instance. If you remain dissatisfied
and would like to request an internal review, then please contact us at the address
above or by emailing email@example.com.
Please note that you have the right to an independent review by the Information
Commissioner’s Office if a complaint cannot be resolved through the PHE
complaints procedure. The Information Commissioner’s Office can be contacted by
calling the ICO’s helpline on 0303 123 1113, visiting the ICO’s website at www.ico.org.uk
or writing to the ICO at Wycliffe House, Water Lane, Wilmslow,
Cheshire, SK9 5AF.