This is a real lesson in LFTs not working well as "tests to enable". Lessons from two outbreaks in sporting teams. Clear evidence that the undetectable infectious period can be longer than hoped, and -ve tests increase risk by disinhibition.

It starts with one Athelete testing LFT-negative on Day zero and then LFT-positive on Day 1.
By the end of the outbreak 32 athletes and staff were infected. 18 of them had false negative antigen test results.
The authors stated that the LFT false negative results led to another 13 people getting infected
The second outbreak happened through a two day competition. The index case was positive on LFT two days afterwards. 12 athletes (5 from one team and 7 the other) were infected in total.
The authors comment on the risks of negative results resulting in "disinhibition behaviour" - they documented multiple instances where rules were broken - which will have increased viral spread.

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More from @deeksj

7 Mar
Sorry @sbfnk I don't see how this analysis takes us forwards. We need to know the +ve predictive value, not specificity, and we cannot get that without verifying LFT+ves with PCR. Your analysis only tells us that the specificity must be =< observed total +ves, not by how much.
Last week total +ves in staff and students was 0.07%, so we know specificity was >=99.93% (we don't need to do any maths to conclude that). So we know it is between 99.93% and 100% but have absolutely no clear where. Without that knowledge we cannot compute the PPV.
If it was 99.93% that means none of the +ves were real cases. If it was 100% then all +ves would be real cases. So we can't tell whether the PPV was 0% or 100% or where it is inbetween. No maths will solve it, only PCR verification.
Read 5 tweets
1 Mar
Testing in schools has been finding close to zilch!

Just located the Test and Trace figures for testing in schools.

They are on this webpage
gov.uk/government/pub…

In Table 6 of this downloadable file

1/6
To interpret these results remember all tests give false positives (FP). For Innova DHSC says about 3 per 1000 (0.3%), were FP in Liverpool it was about 1 per 1000 (0.1%). So only event rates above these figures indicate that a test is usefully detecting real cases

2/6
First Primary schools (presume teachers and staff)

2,031,296 tests; 4,232 positives - that is 0.21% - stable for the past month. This is below the 0.3% false positive rate stated for Innova - so many will be false positives.

3/6
Read 6 tweets
24 Feb
Risks of false positives in school testing with Innova

Innova testing of asymptomatics in England has very low rates of test positivity at the moment (0.3%). The lower this figure gets, the more likely a positive result is to be a false positive than a true positive.

1/7
This rate is in health care workers and others getting community testing – you will expect rates in school testing to be lower as children have been locked down.

What are the chances of a positive result being a false positive?

Best data we have are from University testing

2/7
Kudos to @PubHealthScot for looking at this with University students and clearly and fully reporting their findings.

Excellent weekly reports are on their website
beta.isdscotland.org/find-publicati…

Report from 27th Jan shows key data for Uni testing in Nov/Dec.

3/7
Read 7 tweets
21 Feb
What is the evidence supporting the Government’s claims that Innova LFT detects infectious cases?

There are no real studies at all which directly show how well Innova +ve detects infectious people and Innova -ve indicates non-infectious people.

No direct data at all.

1/19
The data come from studies of PCR with inferences made by linking PCR proxy “viral load” Ct values to the viral load levels we know that Innova detects.

Ct values are not standardised and depend on how much biological matter on a swab. They have high measurement error

2/19
Evidence on infectiousness comes from studies linking Ct values with rates of (a) secondary cases and (b) the ability to culture the virus. Neither are perfect – but are the best we can do. They will both most likely underestimate infectiousness.

3/19
Read 20 tweets
21 Jan
@TomChivers has taken on the challenge of trying to understand the disagreements over LFTs. We had a good chat, but unfortunately the final article mispresents some of my views. I explain more below.

What Covid tests can we trust? @UnHerd unherd.com/2021/01/what-c…
Read 7 tweets
18 Jan
Context matters … I am aware that people outside the UK are reading my tweets and papers, and influencing their thinking about lateral flow testing. It’s really important that you understand the context I am writing from, and why it probably is different where you are.

1/11
The situation and issues in the UK (particularly in England), is almost certainly unlike where you are. The UK Government are unique in many ways and that is impacting on how tests are being used. (As a Brit I am also prone to understatement and being overpolite).

2/11
Our Government have staked their reputation on the Moonshot idea “to get our lives back to normal” using LFTs before there was evidence to see whether it would work. It doesn’t make sense to decide policy before evaluating the technology. Decision-making is now politicised.
3/11
Read 11 tweets

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