Lateral flow tests are only accurate at diagnosing coronavirus when administered by trained professionals, studies have repeatedly shown.
The tests, which give results in as little as 15 minutes, use swabs of the nose or throat. Samples are then mixed in a testing liquid and put into a plastic cassette which can detect the presence or absence of coronavirus and then produce an image of a line, the same way as a pregnancy test, to indicate whether it is positive or negative.
The Department of Health and NHS are instructing people to use the tests on themselves, despite manufacturers of some kits saying they shouldn’t be used as DIY swabs.
Both the swabbing procedure and the use of the test cassette can easily be done wrong and affect the accuracy of the test.
If the swab isn’t done for long enough, or deep enough into the nose or throat, it may not pick up fragments of virus. Medical professionals are also able to use nasopharyngeal swabs, which go right to the back of the nostril, whereas this is not advised for people who test themselves.
And if the sample isn’t properly inserted into the cassette the result might be wrong, or people may misread the display when it produces a result.
SELF-TESTING CUT ACCURACY FROM 79% TO 58%
A University of Oxford and Public Health England evaluation of the Innova lateral flow test, which is being widely used in the UK, found its sensitivity – the proportion of positive cases it detected – fell from 79 per cent to 58 per cent when it was used by untrained members of the public instead of lab experts.
Based on this evaluation, officials pushed ahead and used it for a real-world self-testing trial.
PILOT IN LIVERPOOL FOUND FEWER THAN HALF OF POSITIVES
When the same Innova test was trialled on members of the public in Liverpool – with people taking their own swabs and trained military staff operating the tests – the swabs picked up just 41 per cent of positive cases.
In the study the rapid tests detected 891 positive results, compared to lab-based PCR swabs that found 2,829 positives in the same group. This means 1,938 people got a wrong negative result from the rapid test.
The study didn’t compare this to professionally done rapid tests, but the manufacturer Innova claims its test is 95 per cent sensitive in lab conditions.
…BUT TESTING DONE BY MEDICS IN SLOVAKIA ‘REDUCED INFECTIONS’
Despite rapid lateral flow tests getting bad press, officials in Slovakia used them on 5.2million people – almost the entire population of 5.5m – in a trial that a study later estimated to have cut the country’s infection rate by 60 per cent.
The tests used were between 70 and 90 per cent accurate and all the swabs and evaluations were carried out by trained medical workers. They used deep nasopharyngeal swabs, that go to the back of the nose, whereas self-testing generally relies on a swab of only the nostril.
London School of Hygiene & Tropical Medicine researchers said that the scheme successfully weeded out coronavirus cases that wouldn’t have been found otherwise, slashing the number of cases by over half in a week during a lockdown.
HOW RAPID TESTS ARE DIFFERENT TO LAB-BASED PCR SWABS
Lateral flow tests are an alternative to the gold standard PCR test – known scientifically as polymerase chain reaction testing – which is more expensive and more labour-intensive but more accurate.
PCR tests also use a swab but this is then processed using high-tech laboratory equipment to analyse the genetic sequence of the sample to see if any of it matches the genes of coronavirus.
This is a much more long-winded and expensive process, involving multiple types of trained staff, and the analysis process can take hours, with the whole process from swab to someone receiving their result taking days.
It is significantly more accurate, however. In ideal conditions the tests are almost 100 per cent accurate at spotting the virus, although this may be more like 70 per cent in the real world.