Information relating to PCR test: FOI release

Information request and response under the Freedom of Information (Scotland) Act 2002

Information requested

You provided a document from the CDC in America stating a synthetic flu was used to calibrate their pcr test for covid 19, asking whether Scotland's genealogical team did the same?

You also asked has sars2 covid19 even ever been isolated in Scotland through any means?


1. How do you know that the RT-PCR test for SARS-CoV-2 is accurate?
These tests being used are real-time RT-PCR assays which are used to identify the causative agent of COVID-19 disease, known as SARS-CoV-2 virus, in clinical samples. All these assays have been designed to target a section of the SARS-CoV-2 genetic material which is specific to the virus. There are a variety of RT-PCR tests currently in use in Scotland to confirm the presence of the virus SARS CoV-2, all of which are regulated and approved by the Medicines & Healthcare products Regulatory Agency (MHRA), and thereafter validated for use by the Scottish laboratory performing the test. Furthermore, many of the commercial assays are used as the primary method for diagnosis by a number of other countries world-wide. MHRA Guidance for patients, the public and professional users: a guide to COVID-19 tests and testing kits is available here.

2. What is the sensitivity/specificity of the Scottish PCR test?
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.

3. If a test isn’t 100% sensitive/specific, does this mean we cannot rely on the results it provides?
It is extremely rare for a laboratory test to be 100% sensitive and 100% specific. Using real-time PCR is best technology for detecting respiratory viruses and is the gold standard against which other tests are measured. The analytical sensitivity of a real time RT-PCR is not the only factor to be considered when testing clinical samples. The quality and timing of the clinical sample also need to be taken into account, particularly when the amount of viral material present in a patient sample will change during the course of infection. The tests in use are analytically very sensitive but the timing and quality of clinical samples are important determinants of clinical diagnostic sensitivity.

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