PCR test thresholds for vaccinated and unvaccinated

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Dear Public Health England,

The CDC changed the requirement for the cycle threshold count (CT) those that are vaccinated, <28 CT, COVID positive. For the non-vaccinated <40 CT, COVID positive.
Is this the case in the UK?
Can you also give me the accuracy figures for lateral flow tests

Yours faithfully,

Paul Price

FOI, Public Health England

OFFICIAL

Dear Paul Price,

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Public Accountability Unit
Public Health England
[PHE request email]
www.gov.uk/phe Follow us on Twitter @PHE UK

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FOI, Public Health England

OFFICIAL
Dear Paul Price,

Public Health England (PHE) partially holds the information as specified by your request.

Oxford University and PHE evaluation confirmed lateral flow tests show high specificity and are effective at identifying most individuals who are infectious.

The specificity of the test was recorded as 99.68% - the overall false positive rate was 0.32%, although this was lowered to 0.06% in a lab setting.

It has an overall sensitivity of 76.8% for all PCR positive individuals but detects over 95% of individuals with high viral loads, and minimal difference between the ability of the test to pick up viral antigens in symptomatic and asymptomatic individuals.

You can view this here: https://www.ox.ac.uk/news/2020-11-11-oxf...

Under Section 16 of the Freedom of Information Act, a public authority has a duty to provide advice and assistance. Accordingly, please see the below information on Cycle thresholds.

There are multiple tests being used and every kit will have its own interpretative criteria and thresholds, as recommended by the manufacturer. There are at least 80 different platforms in use, configured in various ways in different laboratories. Some test provides Ct values some does not. Range, mean, median for thresholds will be dependent on the operating arrangements in each laboratory and relate to each kit/assay being used. These laboratories have a statutory duty to report positive cases to PHE, but they are not obliged to advise PHE which tests they are using nor submit CT values used to PHE.

Cycle threshold (Ct) cut offs for covid-19 PCR tests are subjected to a standardised laboratory algorithm and this informs interpretation of positive or negative results. The algorithm is based on evaluating the limit of detection for each test using known positive control material and will differ depending on the assay used.

These data are available from the commercial manufacturer of the assay and is subject to local validation by the laboratory using the test and before using it for diagnostic purposes. The laboratories have a statutory duty to report positive cases to PHE, but they are not obliged to advise PHE which tests they are using nor submit CT values used to PHE. All testing laboratories are also expected participate in external quality assurance schemes and their quality is thus monitored.

Further information on understanding cycle threshold (Ct) in SARS-CoV-2 RT-PCR can be found at the following link. https://assets.publishing.service.gov.uk...

Furthermore, PHE does not mandate what each lab needs to do - we provide guidance - the assessment of quality is performed by UKAS (UK accreditation service). Should you wish to contact them regarding accreditation you can do so at: [email address] or you can view their accreditation page at: https://www.ukas.com/find-an-organisation/

Public Accountability Unit
Public Health England
[PHE request email]
www.gov.uk/phe Follow us on Twitter @PHE UK

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