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Several media reports suggested that changes to COVID-19 testing rules were to be announced by the government, which has now been confirmed by the UK Health Security Agency (UKHSA).
Comments in response to the UKHSA announcement:
Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:
“Today’s announcement by UKHSA is clear except in one respect. Data from the Omicron era were not cited in support of the UKHSA statement that: “Rapid lateral flow tests are most useful at identifying COVID-19 in people without any symptoms. The tests are over 80% effective at finding people with high viral loads who are most infectious and most likely to transmit the virus to others.”
“Pre-Omicron, data are in the public domain – for Alpha and Delta – for INNOVA and Orient Gene respectively from dual testing against PCR (for diagnosis of infection; and against low cycle-threshold (variously defined) PCR as proxy for “infectiousness”). The good news is that in the Omicron-era, from mid-December 2021 or earlier, a dual-testing evaluation has been underway in England for FLOWflex (and possibly, separately, for the above two LFD-types as well).
“For transparency, results for these omicron-era dual-testing evaluations should be placed in the public domain at the earliest opportunity.”
Dr Hayley Jones, Senior Lecturer in Medical Statistics, University of Bristol, said:
“At current high levels of infection in the population, it’s sensible to assume you’re infectious if you have a positive lateral flow test result, without a “confirmatory” PCR. But it’s crucial to remember that the reverse is not true: a negative lateral flow result doesn’t guarantee that you don’t have COVID-19 or that you’re not infectious, so it remains important to get a PCR test if you have symptoms regardless of a negative lateral flow result. Please also report positive lateral flow test results on https://www.gov.uk/report-covid19-result to help track the spread of the virus.”
Comments in response to media reports before UKHSA confirmation:
Prof Sanjeev Krishna, Professor of Molecular Parasitology and Medicine at St George’s, University of London, said:
“With high prevalence of COVID-19 in the community, the importance of false positives becomes lower. In any case, it is likely that repeating a lateral flow test can manage the incidence of false positives and act as a confirmatory test by itself.
“The larger issues here seem to be that tests are in short supply despite the UK Government having encouraged manufacturers to manufacture in this country. One of the reasons is the certification system has failed to recognise tests made in this country, despite the tests having satisfied requirements of other stringent regulatory authorities. This is clearly anomalous and should be looked into in more detail.”
Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:
“There are two important down-sides if PCR-confirmation is not required. Firstly, there is a loss of information on both cycle-threshold value (which acts as proxy for viral load) and genomic variation (which tells us about new variants) as the PCR tests can analyse these whereas lateral flows do not.
“Secondly without PCR confirmatory tests there could be a loss of timely information on positive diagnoses if those who test LFD-positive are less inclined to report their positive-test-result because there is no follow-up action to be taken (ie PCR booking).
“A third consideration, about which there may be empirical evidence for each of the three types of LFD-kit in general circulation, is the extent to which self-testing by LFD gives rise to results as follows: “LFD-positive; same-day/next-day PCR positive but next-day LFD negative” versus “LFD-positive; same-day/next-day PCR-positive & next day LFD positive” versus “LFD-positive; same-day/next-day PCR-negative & next-day LFD-negative” versus “LFD-positive; same-day/next-day PCR-negative but next-day LFD-positive”.
Prof John Edmunds, Professor in the Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, said:
“This change makes a lot of sense. When the prevalence is high – and it is incredibly high at the moment – almost everyone who tests positive with a lateral flow test will be a true positive. There is really no need to confirm this with a PCR, a step that not only wastes time but costs a lot of money and uses up laboratory resources that could be better used elsewhere. There are down sides to this change. We would have slightly less information on the relative prevalence of the different variants circulating in the community, as PCR swabs undergo genotyping and sequencing, and the daily number of confirmed cases may need more careful interpretation. Overall, though, I think that these downsides are worth it.”
Prof Lawrence Young, Virologist and Professor of Molecular Oncology, Warwick Medical School, University of Warwick, said:
“This is a sensible approach as long as positive results are reported and the lateral flow kits are in good supply. Lateral flow tests can detect around 95% of people who are at their most infectious. There appears to be a shorter time from infection to becoming infectious with the omicron variant compared to delta which is contributing to the rapid spread of omicron. For those who test positive on lateral flow but do not have symptoms, it make sense to begin self-isolation without the need for a follow-up PCR – this will mean less time in isolation and less pressure on the PCR testing system. Perhaps this approach should also be considered for those with symptoms, particularly as many folk are experiencing mild, cold-like symptoms – although it is likely that those with symptoms will remain infectious for longer and therefore need a more time in isolation.”
Prof Sir David Spiegelhalter, Chair, Winton Centre for Risk and Evidence Communication, University of Cambridge, said:
“This seems a sensible move. We argued back in October that it seemed fairly pointless to do a confirmatory PCR after a positive lateral flow test.1
“This is even more true now: with current infection rates, a positive LFD followed by a negative PCR would still mean it was very likely that you were carrying the virus. PCRs are useful for surveillance, sequencing and have legal status, but if we want to focus on isolating people who are infectious, then LFDs may be adequate. But they need to be reported.”
Dr Alexander Edwards, Associate Professor in Biomedical Technology, University of Reading, said:
“There has been a lot of discussion about the accuracy of lateral flow tests, and we now have a lot of data that shows how well they work. In spite of this clear data, it can be complicated to understand the statistics.
“False positive results happen quite infrequently with these lateral flow tests, although there are situations when these false positives can be a problem. When covid-19 cases were very low, and schoolchildren started regular testing, some children likely missed school unnecessarily due to false positives. At that time, it made sense to use PCR confirmation.
“But right now, with a very large proportion of the population infected, the number of true positives totally dwarfs these few false positives. In other words, at the moment, if you see even a faint positive by lateral flow, it is highly probable that you do have covid-19. The motivation for the UK system still requiring PCR confirmation more recently is likely to do with monitoring virus variants, rather than confirming the original lateral flow result.
“Given the extreme pressure on the PCR testing service, it makes a lot of sense to skip this confirmation.
“PCR testing remains more accurate than lateral flow, because much lower levels of virus on a swab can be detected. Lateral flow tests still have significantly higher rate of false negatives, and no tests are 100% reliable at ruling-out infection. People should remember therefore that a negative test can’t ‘prove’ someone isn’t infected.”
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
Declared interests
Prof Sanjeev Krishna: “paid consultancy roles with Mologic and QuantuMDx, as well as being a shareholder in QuantuMDx.”
Prof David Spiegelhalter: “No COI , except Non-Exec of UK Statistics Authority.”
None others received.