Lateral Flow Tests: Britain’s Bridge to Freedom?


The leaking of “Operation Moonshot” last summer seems a long time ago now. Before the arrival of the vaccine, there appeared to be no end in sight of Covid restrictions. The government cooked up an optimistic plan to spend up to £100bn on ramping up the delivery of tests allowing Britain’s economy to keep moving, with the use of Lateral Flow Tests (LFTs) a key part of this. However, the arrival of a vaccine changed all of this: as long as we avoid vaccine-escape variants, Britain’s pandemic exit will be via vaccine-acquired herd immunity rather than a mass testing programme. Yet the British government has ordered 383 million Lateral Flow Tests at the expense of around £1.3bn with further purchases expected. As Boris Johnson slowly starts to unlock the country, LFTs will be deployed widely to suppress the virus as the nation undergoes a vast vaccination programme, but the use of LFTs have been under savage attack for their low accuracy.

With the vaccine usurping mass-testing as the government’s exit strategy, millions of LFTs are now being deployed as a bridge to reduce infections in the spring before the vaccine-fuelled summer of fun. LFTs are cheaper than the classic PCR tests which require a laboratory to examine the sample. This allows for the scale needed to test asymptomatic members of the community, such as ‘cluster-busting’ or testing in more-risky settings such as when schools return on 8th March. On the other hand, they do not have the same accuracy as PCR tests, with varying figures of sensitivity from 46% to 70% meaning 30% to 54% of Covid-carriers are slipping through the net.

LFT proponents see them as a game changer allowing frequent tests for asymptomatic individuals, catching infections before symptoms appear and cutting out the pre-symptomatic spread that has been the virus’s trump card throughout the pandemic. This suggestion has created fierce debate within the academic field with briefings and counter-briefings to journalists around the utility of such tests. Professor Jon Deeks, a biostatistician from Birmingham University, has led the doubters claiming that the low sensitivity will provide individuals with false confidence making the tests dangerous. Alongside this, is a concern that testing asymptomatic accentuates the issue of specificity (false positives). LFTs have a specificity of 99.68% meaning 0.32% of positive tests can be expected to be false alarms. This at first appears very accurate however when mass testing asymptomatic individuals this runs into problems. Throughout February, millions of LFTs have been deployed with a positivity rate of around 0.35% suggesting false positives are making up the vast majority of positive results. This causes unnecessary disruption as clusters are closed, isolation is enforced despite there being no true infection. Professor Deeks and co pose that LFTs miss most true positives and are catching many false positives, performing only harm and should therefore be scrapped.

This has been seen by many others as dogmatic and taking LFT results out of context. Proponents reckon that the public recognise the difference between the results of PCR tests and LFTs, a positive result on an LFT should be clarified with a more accurate PCR test and a negative result on an LFT should not be an “all clear” but instead a “not positive”. That is to say, the public should recognise a positive LFT should be double checked and a negative LFT should be taken with a pinch of salt avoiding false confidence. It is also important to recognise that the results LFTs miss correlate with lower infectiousness of the individual. LFTs test positive when an individual’s viral load is above a threshold, this correlates with infectiousness, meaning an LFT misses cases in those that are not yet (very) infectious, or in those who have had the virus but are no longer infectious. With frequent testing LFTs should be able to catch when an individual is infectious and therefore break up chains of transmission.

They may not be a silver bullet, but LFTs will be key to restore some normality to Britain before widespread vaccination. The mass testing regime will allow children to receive a proper education, care home residents to receive visitors, and allow Britain to clamp down on any suspicious variants as they arise. They will need to be deployed alongside careful messaging, noting the low sensitivity and a policy of double-checking a positive LFT with a PCR test should nullify concerns around their accuracy.

Image: Medakit Ltd on Unsplash

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