Section Two - What Tests We Use
There are two types of test for COVID-19 currently in use in Scotland: viral (PCR) testing and antibody testing. PCR tests are used to detect if someone is currently infected with the virus; and antibody testing is used to tell us if someone has had the virus.
PCR tests, used to test for current infection, operate as a swab taken from the nose and back of the mouth, with the sample collected sent to one of the existing NHS Scotland laboratories, or the Glasgow Lighthouse Laboratory, to be analysed. Any positive cases identified are automatically followed up for contact tracing through our national system of Test and Protect.
Antibody testing is used to test for past infection. In Scotland currently, it is used to track what proportion of the population has already been exposed to the virus. We don't yet know whether people who have had the infection are immune and cannot get infected again, nor how long any immunity, if proven, may last. Until this evidence base develops, our current policy is to use antibody testing for population surveillance purposes, and in limited clinical scenarios.
In addition to PCR testing, and antibody testing, Scotland has world leading research expertise in viral genomics. Genome sequencing of the COVID-19 virus is currently being undertaken by a Glasgow and Edinburgh partnership working as part of the COVID-19 Genomics UK (COG-UK) Consortium.
Whole Genome Sequencing contributes to our understanding of how the disease moves through the population and changes over time. In particular it can improve our understanding about whether cases are likely to be linked or not. As rapid sequencing is now being delivered in Scotland (with results available within 48 hours of a sample arriving at the appropriate laboratory) it has the potential to play an important role in providing information to support the management of outbreaks. Whole genome sequencing can also show geographic links – and help us understand what region or country that virus emerged from.
Limitations of testing
No test is perfect, and understanding the limitations of the tests we currently use is important. If we assume tests are perfect, and that results always accurate, we put others at risk.
In PCR testing, the key risks are false negative results – where a test is negative but the person tested does actually have COVID-19 and is infectious – and occasions where the test is positive but the person tested is not infectious.
False negative results can happen if a swab misses collecting cells infected with the virus, or if virus levels are low – for example, at the start of an infection. The risk to others of false negative results is clear – an infectious person who receives a negative result risks transmitting the virus to others, including vulnerable people who can suffer very severe harm. In certain situations, testing again after a number of days – when levels of the virus may be higher and therefore detectable – can reduce the risk of false negative results having serious consequences.
Weak positive results can happen when the swab picks up fragments of the virus from an individual who is no longer infectious. Laboratories in Scotland have now implemented confirmation testing (or repeat testing) in certain circumstances to confirm whether weak positive test results are actually infectious cases.
Opportunities of testing
Understanding the limitations, and applying the principles for the use of testing as advised by our expert scientific advisory groups, in the context of our overarching pandemic strategy, means we can use Scotland's enhanced capacity for testing in optimal and agile ways in the next phases of the pandemic.
If we know the limitations of PCR testing are higher where prevalence is low, and we know our overarching strategy is to drive the number of cases to a low a level as possible, then we know the opportunity is to use our capacity now to actively find cases where they are most likely to be, and where they are likely to do most harm, while simultaneously building our capacity for winter when there will be a growing need to distinguish COVID from other common respiratory illnesses.
We will also actively monitor developments around testing innovation so we can take advantage of any new opportunities from testing they present.