COVID: how to make sense of the UK’s new testing rules
COVID: how to make sense of the UK’s new testing rules
One positive aspect of the pandemic has been increased public understanding of epidemiology and even virus biology. However, there still seems to be significant confusion when it comes to COVID tests – particularly when or why each type of test can or should be used.
Recent changes to the UK’s testing rules have only added to this problem. As of early January, the UK government has reduced situations where it is necessary to take a PCR test in favour of greater reliance on lateral flow tests (LFTs).
Without a clear explanation, such changes risk seeming rapid and arbitrary, with the suspicion that they are based on just saving money. However, when assessing the strengths of each type of test, together with the present state of the pandemic in the UK, can we discern any scientific logic behind these changes?
Understanding the tests
PCR tests detect the presence of the coronavirus’s genetic material (RNA) in a sample, and they are extremely good at doing this. But they do not show whether someone is currently infectious. A person may have recovered from COVID but still have small amounts of the virus in their system – hence recommendations not to repeat PCR testing within 90 days of a positive result.
LFTs instead measure whether viral proteins (called antigens) are present in a sample. They are only positive when a sizeable amount of virus is present. They therefore give a very good indication as to whether someone is currently infectious.
This difference between detecting the presence of the virus and whether someone is likely to be infectious is critical for deciding which test to use in any given situation. For instance, the recommendation to use LFTs twice weekly (or daily for some professions), regardless of feeling ill, makes absolute sense when trying to stop people who may have COVID – but no symptoms – from inadvertently spreading the virus.
On the other hand, if the intention is to confirm that someone definitely has COVID – for instance if they have cold-like symptoms that could potentially be caused by other bugs – then conducting a PCR test makes sense.
This difference can be further understood if you compare the tests performance over the course of a COVID infection. As a PCR test is about 1,000 times more sensitive than an LFT, it can detect COVID one or two days prior to the infectious period, and then for quite a long period afterwards as the body destroys the last elements of infection. LFTs tend to only give a positive result during the infectious period.