- 1 Mar 2021
Date received: 2 Feb 2021
Date responded: 15 Mar 2021
"I am requesting furter information to points 3 and 5 of FOI/202000106118
Point 3 > why is a a Cycle Threshold of around 40 amplifications being used?
Point 5 > what is the percentage of accuracy of those without symptoms?"
On 21 December Public Health Scotland published an updated version of Covid-19 Laboratory Testing - Frequently Asked Questions. You can find it here: HPS Website - COVID-19 - laboratory testing frequently asked questions (scot.nhs.uk)
The sensitivity and specificity values depend on a number of factors, including the assay used, the population sampled, how the sample is taken and stored, type of sample and stage of infection. These factors are variable at different periods of time, and across different laboratories and regions of Scotland. It is therefore not possible to give one percentage value which is representative of the sensitivity or specificity value of the PCR test in Scotland.
There is a reflex testing procedure in place for Scotland which was put in place to reduce the likelihood of false positive results. Refer to Guidance for sampling and laboratory investigations for more detail (COVID-19 Guidance for sampling and laboratory investigations). Public Health Scotland do not hold data on false positive/false negative results.
The Cycle Threshold, or CT value, is the number of PCR cycles that it takes before the virus is first detected; the lower the CT value the higher the level of virus in the original sample. Each manufacturer of the PCR test will recommend a differing maximum amplification cycle number to
determine presence of SARS CoV-2 when interpreting results, but a maximum of around 40 amplification cycles is normally used.
Public Health Scotland have published frequently asked questions about COVID-19 laboratory testing more information can be found at HPS Website - COVID-19 - laboratory testing frequently asked questions (scot.nhs.uk)
The Cycle Threshold, or CT value, is the number of PCR cycles that it takes before the virus is first detected; the lower the CT value the higher the level of virus in the original sample. Each manufacturer of the PCR test will recommend a differing maximum amplification cycle number to determine presence of SARS CoV-2 when interpreting results, but a maximum of around 40 amplification cycles is normally used.
Public Health Scotland’s frequently asked questions may also be helpful for you. Based on Local data published, PCR is 98% accurate for those with or without symptoms.
No test is 100% accurate. The sensitivity of a test is the proportion of people who have the condition that is being tested for who actually have the disease. This gives the true positive rate and by extension defines the false negative rate (those who should have shown positive but the tests failed to see them as positive) Specificity is the opposite to sensitivity; it identifies the true negative rate and thus the false positive rate. Sensitivity and specificity define the accuracy of a test – how well does it rule in or rule out the diagnosis.
The type of tests being used for diagnosing covid-19 is one that detected the viral genetic sequence of the causative virus SARS-CoV-2 by a technique called real time PCR assays. These tests are very sensitive and the gold standard for respiratory viruses. They are specific and shown not to detect other coronaviruses and have been tested on large panels of negative clinical samples.
The current PCR tests in use in Scotland are effective at identifying people who have COVID-19 infection when they are symptomatic. As these tests only detect the presence of RNA from the SARS CoV-2 virus they cannot distinguish between live and inactivated virus. As a result they cannot tell us if a person is currently infective. This means that testing cannot reliably tell us if someone who does not have symptoms currently has the disease, or has had it in the past and has inactivated virus in their sample.
The Scottish Government is committed to publishing all information released in response to Freedom of Information requests. View all FOI responses at http://www.gov.scot/foi-responses.
Please quote the FOI reference
Central Enquiry Unit
Phone: 0300 244 4000
The Scottish Government
St Andrews House