Lateral flow tests still have a vital role in the UK’s fight against Covid

Infectious disease modelling shows that if everybody could do a coronavirus test twice a week and self-isolate if positive, the pandemic would collapse. Capacity for widespread PCR tests, which look for genetic material from the virus in samples, was built up through the UK’s Lighthouse labs, but initially limited to people with symptoms. Because they are relatively expensive, equipment-intensive laboratory tests, turnaround was often several days, by which point it would be too late to prevent onward transmission.

Then along came the apparent godsend of the cheaper lateral flow tests (LFTs) that use a swab twirled in both nostrils and the throat, seeking coronavirus proteins that bind to antibodies on the stick, and give an answer in 30 minutes. Epidemiologists suggested that, if combined with adequate income support for self-isolation, this could be a critical tool in Covid control. Six months later LFTs are still controversial, with some saying they are “a complete waste of money”.

The first round of complaints about the LFTs, when the virus prevalence was high late last year, was that they missed too many cases compared with the PCR test and thus provided false reassurance. It is now clear that many people who test positive by PCR do so many days or even weeks after they have ceased to become infectious. The LFTs on the other hand detect 80-90% of cases in the days when people are most infectious, and then rapidly become negative when they are unlikely to transmit the virus. This characteristic made it look like the LFTs gave a lot of false negatives compared with PCR – but from an infectiousness point of view, it is the latter that is actually giving a lot of false positives.

With the virus prevalence much lower, the criticism now is that a high proportion of the LFT positive results are false positives. It is a characteristic of any test that produces occasional false positives that when the number of true cases in the population is low, a higher proportion of the test positive cases will be false positive. Thus, if one in 1,000 LFTs are graded as a false positive, when the prevalence of the virus is one in 100 – and if LFTs detect 80% of these – there would be one false positive and eight true positives detected.

The Office for National Statistics report on 16 April put the prevalence at about 1 in 500 in England and Scotland. This should mean that now for every 1,000 people tested, we should see 1 false positive and 1 or 2 true positives. Critics complain that too high a proportion of test positives have to be unnecessarily asked to self-isolate when the virus prevalence is low.

The obvious solution is to quickly do a confirmatory test. This could be PCR, but it could probably also be another on-the-spot LFT. Technology may eventually help out, as many of the false positives appear to be mistaken or inexpert reading of the lines indicating positivity on the LFTs, and the development of smartphone apps that use artificial intelligence to process a photo of the LFT may considerably reduce the number of false positives reported.

The UK government has now agreed that a negative PCR test taken within two days of a LFT positive releases someone from self-isolation. There are still problems with the test, trace and isolate and Lighthouse laboratory systems, but turnaround has improved. If the private sector is used it needs to be better regulated: I have personal experience of not receiving a test in time to be useful from a company on the government’s list of approved providers.

LFTs, combined with rapidly available confirmatory tests are important tools in Covid control. They are not, however, the “cheap and cheerful” alternatives to laboratory tests they are sometimes depicted to be. They are actually quite sophisticated medical devices, and care has to be taken to evaluate the offerings from different manufacturers and monitor their performance over time and from batch to batch. Just like the PCR tests, a change in the chemical reagents used in their manufacture can lead to poorer performance. Fortunately, the government has established testing for these kits at its Porton Down laboratory, whereas many other countries, including the US, rely on test performance being reported by the manufacturers.

So would I feel safer on a crowded plane in which everyone had tested negative by PCR or by an LFT at the airport? The easy answer is that I would certainly feel better in either scenario compared with doing nothing – and the same applies to buses, trains, indoor dining, theatres, sports events, hospitals and any shared workplace. Given the cost of PCR tests, failing to take advantage of the lower cost and quicker turnaround of the LFTs is often the equivalent of doing nothing. And the full answer is that that both approaches might miss some cases. A PCR done three days prior to a flight will not detect someone who turns infectious in those three days. The LFT is up to date, but may miss some people with low viral loads.

As a passenger however, I would much rather take the very small risk of being turned down at the airport because of a false positive LFT than face an anxious wait wondering whether the provider of my PCR test would get the result back to me in time. If admitted to hospital, I would feel reassured that twice-weekly LFT testing of NHS staff has resulted in tens of thousands of potentially infectious staff remaining at home rather than coming to work.

For population-wide screening, we will never be able to build enough laboratory capacity to test a high proportion of the population twice a week by PCR one sample at a time. If we could do so with LFTs, now provided for free by the government, and optimise the PCR capacity to provide rapid confirmatory testing (that will also allow us to monitor for new virus variants), we would put a substantial part of control of the epidemic in the hands of the people.

Some of the same critics who were sceptical of our compliance with lockdowns think we will not follow through on self-testing. However, public engagement remains high. Effective use of LFTs, along with vaccines and social distancing, should help us get closer to the goal of a zero-Covid UK and to keep infection rates as low as possible.

• David Hunter is the Richard Doll professor of epidemiology and medicine, University of Oxford