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From PMQs to the No.10 press briefings, Boris Johnson has had plenty of tricky questions lobbied his way over the past few months. But the one that really made him squirm most today was from a member of the public who innocently raised the topic of his wedding plans.

In his Facebook “People’s PMQs”, wedding organiser Lara said “you’re supposed to be getting married too..” Perhaps it was the phrase “supposed to be” that unnerved him, but Johnson sounded distinctly uncomfortable as he blurted out: “I didn’t want to drag that in…anyway, we’ll take that one offline, as they say.”

As Twitter frothed with wild speculation about whether the PM was or wasn’t getting cold feet about his planned nuptials to fiancee Carrie Symonds, the more interesting point was Johnson’s suggestion on how the wedding industry could revive next year: through lateral flow tests.

There was a “real possibility that lateral flow testing will allow events like weddings to go ahead”, he said. Moreover, it was his strong hope and belief that by the summer “whether by vaccination, which I hope and believe we will have delivered by Easter, or by lateral flow testing, we will be in a different world”.

Read that sentence carefully and you realise just how much emphasis Johnson puts on rapid-testing technology. It’s as important as vaccination in the liberation of the public to do the things they have longed to do, he suggested. In fact, the PM said the phrase “lateral flow test” more times in that PPMQs than any other Covid buzzword. Just as he has all week.

There are lots of good reasons for wanting lateral flow tests to work. Their speed means people can isolate much quicker (and any positive result is always backed up by a PCR test to confirm it). They’re less invasive than PCR swabs and they have potential to change lives.

But, as I’ve previously written, there is actually not a lot of data on the effectiveness of lateral flow device (LFD) tests used in Liverpool or whether their “community testing” pilot really did lead to the dramatic fall in coronavirus cases in the city. The data will come in time, but it’s not there yet, despite Johnson and Matt Hancock linking testing to case numbers.

And today there were fresh doubts raised about lateral flow tests by Birmingham University biostatistics professor Jon Deeks. He seized on a little-noticed stat buried in this week’s community testing prospectus for councils, showing that lateral flow tests in Liverpool actually had a much lower “false negative” rate than lab tests.

“In the field evaluation in Liverpool, compared to PCR tests, these tests picked up 5 out of 10 of the cases PCR detected and more than 7 out of 10 cases with higher viral loads, who are likely to be the most infectious,” the government’s own document states. Deeks said that missing 50% of all cases was bad enough, but missing 30% of high viral loads was “NOT SAFE” [his caps, not mine, reader].

As it happens, even the Oxford/Public Health England evaluation of these tests found their sensitivity under lab conditions (76% overall and 90% for high viral loads) fell to just 58% when put in the hands of self-trained individuals. But there will be members of the public who wonder whether Deeks is onto something to question just how reliable the tests are. Especially when their use is now a key part of plans to let relatives visit care homes.

Insiders at NHS Test and Trace stress that the whole point of the rapid tests is to be additional to, not substitutes for, the “gold standard” PCR tests. They also say that care homes are being opened up through better infection control and PPE, and again lateral flow tests are on top of all that. But only this week Sheffield council said it would not use them in its care homes because of “an unacceptably high risk of not correctly detecting infected individuals”.

It’s not just weddings or care homes that the PM envisages this technology allowing a return to more normal life. Theatres, football matches, airline flights, students, may all come to rely on them under government plans. Yet people like Deeks have long been sceptical about using them in “test and release” settings like universities.‌

Only this week, it emerged that the government wants to pilot ways to allow individuals in Tier 3 areas to have Tier 2-style freedoms to mix with others if they provide to negative LFD tests. And of course they are proposed as part of routine testing of NHS staff too. I understand that one key reason for using them in the NHS is that – despite the 500,000 daily PCR capacity – there just isn’t enough PCR capacity in hospitals to make routine testing work. That doesn’t sound to me like they’re “extra tests, not substitute tests”.

The science on just how many asymptomatic cases can be picked up by LFDs is also unclear. And several academics are wondering just why an independent regulator like the UK National Screening Committee has not been involved in signing off their use in community testing. Vaccines need regulators, as we’ve seen this week, so why not mass testing?

Asked about Deeks’ concerns, the DHSC told me tonight: “With up to a third of individuals with Covid-19 not displaying symptoms, broadening testing to identify those showing no symptoms and who can infect people unknowingly will mean finding positive cases more quickly and break chains of transmission. The country’s leading scientists have rigorously evaluated the Optigene LAMP test and Lateral Flow Test and confirmed the accuracy of the tests for asymptomatic testing.”

There’s another worrying sign for the government’s critics too. The tests used in Liverpool and elsewhere are made by US firm Innova Medical, but supplied to the UK via a tiny company called Tried and Tested, which has (according to the Guardian) no apparent UK office and operates out of a room in Wallingford, Oxfordshire. The £138m contract signed with Innova was not put out to tender.‌

The PM may have some explaining to do to his fiancee Carrie tonight. But he also has some explaining to do to the public to counter claims that rapid flow tests are worryingly unreliable. Johnson seems so wedded to the new tech that he may not break off his attachment. He now needs to spend time making the scientific case for it.