So are Lateral Flow Tests are becoming more useful?
Eh – No
Last week 2,764,845 tests done (new record high) and 4,353 were positive. That’s 0.16% (new record low).
So probably lots are false positives. Lots of money spent– little found
In secondary school 663,332 were tested, only 328 positive. Testing has move than doubled on previous week, numbers detected have not.
We are now down to 0.05% being positive – that is only 1 in every 2000 tested – another record low.
Given what we know about the infection rates from ONS and REACT – these data raise serious concerns that the LFTs are not doing the job that we expect. Many more cases should have been detected.
When will @dhscgov stop being an Ostrich and wake up to the problem?
The mistake being made is to think that all that matters is that false positives are rare. However, if true positives are even rarer then we are in trouble. Test positivity data from Test-and-Trace (1 in 1500 positive) raises serious concerns that is the case.
2/8
If 1 in 100 pupils had Covid-19, we would be fine. Five out of every six test positives would be true positives. But it looks like we are somewhere between 1 in 1,000 and 1 in 10,000 (we don’t know).
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.
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.
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.
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.
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.