Increasing use of lateral flow tests?

Drawing of a man coughing

Another example of rather confusing debate was found in this BBC Inside Health podcast. You’ll find it 15 minutes in.

Lateral Flow Tests - for detecting current infection

a blue-gloved hand holding a Lateral Flow Test

It was about the merits of the cheaper lateral flow tests being used to repeatedly test people multiple times (2-3?) per week.

PCR Tests - for detecting current infection

PCR Swab Testing Chart

PCR tests (the lab-based swab test) are good at identifying people who are actively infectious. But they also identify people as positive who are post-infectious but still carry dead (i.e. harmless) viral particles in their upper respiratory tract. This little appreciated effect can last up to 3-4 weeks after the last infectious day.

In addition, PCRs are only allowed for people who are either worried about a recent exposure or who are symptomatic, so the PCR testing system misses asymptomatic and pre-symptomatic spreaders of the virus. This is a major contributor to the spread of the virus through the community. Further, it’s complicated to arrange the PCR test and it takes 24-72 hours to get the result.

Lateral flow tests look for proteins made by the live virus, unlike PCR tests that look for viral RNA fragments. They are cheap and give quick results. They don’t need a lab. This makes them ideal for regular home and work use. The main problem is that they are not as sensitive as PCR tests and can give false negatives. One reason for this might be the self-tester using poor technique with the swab (lateral flow tests are always swabbed by the person getting tested, whereas a lot of PCR swabs are done by a ‘professional swabber’). Another might be just the nature of the technology in that PCR tests amplify tiny amounts of target material and are just that more sensitive than lateral flow tests.

Arguments

A GP on the program argued that they shouldn’t be used for mass / repeat testing because a negative result from the lateral flow test might encourage people to choose behaviours that they might otherwise have avoided. The GP also made the traditional argument that ‘we shouldn’t do anything until we have evidence’.

The epidemiologist, arguing in favour of mass lateral flow testing, focussed on the gains to be had from these tests picking up asymptomatic and pre-symptomatic cases which would otherwise have been missed. It wouldn’t do it perfectly, but it would probably get half. Half is better than nothing.

These positive test results would then encourage the person to isolate (or at least not go to work).

So the gain from the a lateral flow test rollout would be to permit, in a way that PCR tests can’t, more intelligent adaptive behaviour from these unwitting ‘spread the virus’ groups.

Conclusion?

If these lateral flow tests are used to screen people who don’t have symptoms but are going out into the community or workplace then it seems to me that they will add value.

An example of where they shouldn’t be used is by a person who has Covid-19 symptoms deciding that a negative lateral flow test means they are not infectious. That would be crazy.

A public health message that these types of DIY tests are intended only for non-symptomatic people who are already engaged with family, community or workplace interactions is the right way to go.