Thread by @_johnbye

The full results of the Liverpool LFT trial have been released.

False positives are much lower than expected, but false negatives much worse.

Liverpool has now paused plans to use the tests on care home visitors, following Sheffield and Manchester.

gov.uk/government/pub…

The good news is the LFTs only gave 2 false positives out of 3,199 tests checked by PCR. That gives a false positive rate of 0.07%, far less than the 0.38% reported in initial tests.

The bad news is they missed 23 of the 45 PCR positives, giving a 51% false negative rate!

We already knew the Innova LFTs were less sensitive than PCR, but they were meant to catch 95% of people with "higher viral load".

In the Liverpool trial though they missed 15% of people who were positive at below 20 cycles on PCR, and 47% of people positive at 20-25 cycles!

Overall the trial missed 31% of the people most likely to be infectious. Almost one in three.

It's no surprise councils were angry when the government sent these tests to care homes without telling them, and before the trial results were published.

news.sky.com/story/covid-19…

Before we write LFTs off completely though, results from Birmingham Uni (which has been screening students before they go home for Christmas using the Innova tests, and cross checking some results with PCR) look more promising.

More on that tomorrow...

https://twitter.com/alanmcn1/status/1336775706118922253?s=19

Meanwhile the government is pushing ahead with rolling out 1.6 million LFTs in tier 3 areas this month alone, despite concerns over their effectiveness in asymptomatic testing and the total lack of evidence that they helped reduce cases in Liverpool.

https://twitter.com/DHSCgovuk/status/1337672049788153856?s=19

Picking up from yesterday, how did the Innova lateral flow tests perform at the University of Birmingham?

First the bad news: of 720 random negative tests they checked with PCR, 6 were positive.

Extrapolating to all 7,189 people tested, that suggests about 60 false negatives.

Given they only got 2 positives from the LFTs, that means they might have missed 97% of people who had the virus!

But this doesn't tell the whole story. Unlike in Liverpool, ALL of the false negatives were found by PCR at higher Cts (29+).

So at Birmingham University the Innova lateral flow tests seem to have caught the only two asymptomatic people they tested who were confirmed by PCR to have what the initial assessment classed as "higher viral load".

Which is what they're meant to do.

But this doesn't guarantee people who got false negatives weren't infectious when tested or didn't go on to develop symptoms.

And it's debatable whether we can read anything into such a small sample size - in Liverpool 40% of the false negatives were detected at Ct 25 or lower.

On the bright side, after testing 7,189 people the only 2 positives were both confirmed by PCR at low Cts (20 and 25).

So there were NO false positives, again suggesting the false positive rate for the Innova LFTs is MUCH lower than the 0.38% quoted in the initial assessment.

So where does this leave us?

The tests are clearly far from perfect.

But they're not completely useless.

More than 99% of people got the right result, and in Liverpool they found hundreds of asymptomatic people with high viral load who might not have been detected otherwise.

BUT they may be missing up to a third of people who are currently asymptomatic but have high viral load, and may therefore be infectious.

So using the tests to screen people before visiting care homes (as the government wants to do) seems VERY risky.

manchestereveningnews.co.uk/news/greater-m…

There may be ways they can be used effectively though.

Professor McNally (who ran Birmingham Uni's screening programme and helped setup the first Lighthouse Lab) suggests using them as a quick, cheap way of screening symptomatic people, for example.

https://twitter.com/alanmcn1/status/1335157663072727040?s=20

Even mass screening could be useful IF there's proper support for people who test positive to self-isolate (which isn't currently the case) AND it's made clear that a negative result isn't a license to go wild (ditto).

Catching half the asymptomatic cases could still be useful.

But all of this needs a coherent plan, careful messaging, a functioning contact tracing system, and adequate support for people self-isolating.

Not a hodge podge of different approaches in different towns, and a service that still fails to reach 40% of non-household contacts.