PCR Cycle Thresholds10 Aug 2021 10:21
Freedom of Information Team
Department of Health and Social Care
39 Victoria Street
London
SW1H 0EU
www.gov.uk/dhsc
Mr ---------------
10 August 2021
Dear Mr ------,
Freedom of Information Request Reference FOI-1346345
Thank you for your request dated 14 July 2021 in which you asked the Department of Health and Social Care (DHSC):
“I would like to know what PCR Cycle Threshold (CT) is currently being used to ascertain the number of Covid cases in the UK and whether it is in line with WHO recommendations.
I would also like to know what the CT rate has been historically over the past 18 months and the dates upon which the threshold rate has changed.”
Your request has been handled under the Freedom of Information Act (FOIA).
DHSC holds the information you requested.
A typical RT-PCR assay will have a maximum of 40 thermal cycles. The lower the Ct value the higher the quantity of viral genetic material in the sample (as an approximate proxy for viral load). Ct values obtained in this way are semi-quantitative and are able to distinguish between high and low viral load.
Ct values cannot be directly compared between assays of different types due to variation in the sensitivity (limit of detection), chemistry of reagents, gene targets, cycle parameters, analytical in-terpretive methods, sample preparation and extraction techniques (gov.uk).
While the labs do record CT values for analysed test samples, a data collection exercise is not via-ble currently.
More information on PCR CT values is available here - Cycle threshold (Ct) in SARS-CoV-2 RT-PCR - GOV.UK (www.gov.uk).
With regard to WHO recommendations, it is not clear which recommendations you are referring to. I should point out that whilst the FOIA provides a right of access to recorded information held by public authorities (typically including information such as emails, letters, documents, reports, poli-cies and datasets), it does not require public authorities to generate new information to answer questions, including providing explanations or giving opinions, unless this is recorded information that they already hold.