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Professor David Livermore: (In)accuracy of Lateral Flow Testing

David Livermore, Professor of Medical Microbiology at the University of East Anglia, has been in touch with Lockdown Sceptics to point out a couple of issues with rapid antigen lateral flow testing, which has been trialled recently for use in mass-testing. Seems these tests might not be terribly accurate.

Rapid lateral flow tests for COVID-19 were used extensively in Liverpool a few weeks ago and are now being touted for wider use to “free populations”. But how accurate are they? In Liverpool, 700 positives were found among 140,000 people tested, which had to be set against a prediction of 560 false positives in 140,000 tests, based on the DHSC estimate of 99.6% specificity.

Others have their doubts too, including the American Society for Microbiology (ASM) and the Nevada State Health Department (NSHD). I don’t think these will surprise many reading here. They certainly don’t surprise me. We agree, I think, that there is a fundamental problem with using diagnostic tests in mass fishing trips among the asymptomatic well, in whom the ‘Pre-test probability’ (forgive me, horrible jargon for population prevalence) is low. False positives inevitably become a curse, addressed only (as with the NSHD) if one then does a second confirmatory test on every positive.

What does impress me is that these statements have come from such pukka sources rather than the usual heretics. The ASM (American Society for Microbiology) is very much the premier professional microbiology society in the United States and its statement is endorsed inter alia by the Society for Healthcare Epidemiology in America – again a highly respected professional society – and by the US Association of Public Health Laboratories. Neither is the NSHD to be sneezed at – the fact that it asserts problems with two different tests from different reputable diagnostic companies suggests that the issue is generic, not just one bad test type.

I did like that the ASM piece said: “The cost of frequent, rapid testing would be tremendous. For example, at $5 per test, testing everyone in the US three times per week would cost $4.9 billion per week. When considering an approach of 30 million tests weekly (~10% of the U.S. population), the Rockefeller Foundation estimates a cost of $75 billion in the next year. Even if a $1 test existed, it could cost up to $40 billion in increased spending. Limiting testing to school-aged children, college and university students, full- or part-time employees and other groups who could have frequent contact with multiple people, as well as continuing with medical pre-procedural testing, could still be expected to cost billions per week.  It is unclear where funding would come from.

Actually, this looks almost cheap compared with the wildest fantasies of ‘Operation Moonshot’, which comes in at £100 billion (US$133 billion) for a population of around one fifth that of the USA. That’s about 70% of the UK NHS annual budget… SAGE and the Office for Budget Responsibility might benefit from reading these. Boris and Hancock too.

Stop press: Sheffield City Council has issued an instruction to care homes “not to use lateral flow test kits… until we can gain assurances of their effectiveness” dealing a blow to plans to allow visiting. It told operators that “when used in practice, the performance… appears to show an unacceptably high risk of not correctly detecting infected individuals”. Too many false negatives, in other words. The Guardian has the details.