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GeneGenie- Just to qualify my question: by prevalence, I mean percentage in the population, not concentration or load in any specific test sample. Thank you.
I don’t think Statto can help. I think you’ve answered your own question. The more of the virus the easier to detect in an antigen test, likewise the more antibodies the easier to detect in an antibody test. What’s stats got to do, got to do with it?
Not Tina Turner.
GeneGenie- May I ask a question, please? Statistically, testing accuracy figures are enhanced when the prevalence of active virus or active antibodies is high; have I got this right? Does this mean that, if hoped-for prevalence of active virus drops, the tests for active virus (antigen tests) become statistically less reliable? Conversely, if the prevalence of active antibodies rises, the tests for active antibodies become statistically more reliable ? Would appreciate your professional wisdom, here. Thank you very much.
If we think our test will be widely used (as in daily, at place of work, before entering a stadium/flight), I'm sure false negatives won't be a big issue as it just merely means the viral load is below the Limit of Detection. The person will likely test positive at a later point once the viral load is above this LOD.
Even at 80% sensitivity, someone with the virus getting two consecutive false negatives (20% x 20%) is 4%.
The real question is at what viral load is transmission of the virus happening? Remember people are typically only getting tested when symptomatic, but we know asymptomatic people are also spreading the virus.
174 to sell. Hopefully, we will recover in to close.
GenieGenie, do you realize how arrogant you sound when you say things like that?
Honestly, if you told me we were dealing here with the Collatz Conjecture, maybe even Goldbach's Conjecture?, I would say, dude, we are not in the same league, but we are certainly not.
Tell me again why you think because you are a biostatitian makes you so special from those 95% of LSE? I almost chocked on my oats just now.
And your analogy with Warren Buffett, I guess your specificity example has gone out of the window, as he's really screwed up with his airlines investment, not to mention the Heinz story.
Besides, I am sure with all your alleged diplomas and hours adding up numbers and building pie charts, I am sure you haven't done better than that mole hunter and his 2 million quid, have you?
Regards,
ST
False negatives are more of an issue, but as mentioned there issues with the gold standard PCR due to poor swabs. The advice to isolate if you have symptoms and not to fully trust in tests will always remain. I’m sure Avacta’s test will be huge value to the world.
False negative is someone which has it but tests negative
I've been on four hour conference call - to be honest I'll agree with anything
Not a Michelin chef - just tired
RD, think you meant some false positives will slip through.
False negatives, blimey, that’s another garden of moles.
Not a proof reader.
PCR testing is not without its problems, it can detect remnants of the virus long after the virus has gone
Seriously. You went ahead and had this discussion without.
DrBrix press conference. Described prevalence of 1:1000.
100% accurate for true positive
1% false positive
Test 1001 people
Catch one true positive
Create 1000 x 1% false positive = 10
So for every 1 true positive you isolate, trace etc 11 people.
But if you use this test to extract 11 people for pcr testing you reduce your pcr requirement by 989 people. Furthermore anyone testing negative can carry on. Yes some false negatives will slip through - its a trade off especially if they are in a low risk group
Not an epidemiologist or a statistician. Have a girlfriend to prove it.
Wolfie - agreed. I doubt there will be total focus on any one market, so the rules of one particular regulator aren't too important. I only mentioned FDA approval as a relatively low bar to clear given the recent relaxation of the test requirements in light of the ongoing emergency.
Are you still digging that hole? Banjo will be getting in touch to see if you can him some moles soon.
False positives are of no concern because (a) affimers are highly specific and (b) there is no risk in a small additional percentage of the population needing to isolate.
At no point did anyone say “loads”. Obviously you can take it to the extreme, if the test produced 100% positive results then it would suggest that everyone should isolate. Marvellous for controlling the spread but it’s reputation would be ruined. Avacta’s test won’t suffer this pitfall.
GG, re "from a marketing standpoint a company will sell a test on the basis of its quality, not just that it's been approved (assuming that your audience knows the difference)."
IF (big IF), we focus solely on the US market, FDA approval is these days "more than desirable". My understanding is that without FDA approval, the FDA will check up on the efficacy of your product and if it doesn't meet "what you say on the tin", they will DEMAND that you remove the product from the market.
@GG - I think we are aligned. The reliability of any test is impaired by too many false positive results and in the context of this Pandemic the social impact and consequences for future compliance would also be a consideration. However as stated before - I don't see the test generating false positives due to the High selectivity of the affimer technology as already communicated by the company. It would need to be an existing design artefact of the LFD independent of the affimer detection and we don't seem to be hearing about those so I doubt we have a problem.
Good luck with your investment.
Ophidian
SiriusB, i can say confidently that the PCR is the gold standard. The comparison is post-collection I.e. if two identical samples were tested using different tests, how would they compare to each other. 70% is not the description to look at.
The advantage of Avacta test is its poc and time - the accuracy is important but at post-collection.
Where we stand, Avacta is doing well and I hope it continues to do so by saving lives and in the process making investors money.
@Ophidian thanks for the correction
And I agree, I hope I haven't gotten my point muddled. I believe that the affimer platform and pre-LFT work has shown that the S&S should be well above what is required, and far beyond what most competing tests can offer. However, that remains speculation until confirmed by Cytiva/Avacta. And for people with below-detectable levels - Avacta should only be worrying about what is possible to detect. It's science, not magic.
My main point of contention is people brazenly stating that "it doesn't matter if a test produces loads of false positives".
@SiriusB - I think the Mass Spec BAMS test will soon be the new "Gold Standard". Both Quantitative and HIGHLY sensitive.
Ophidian
Ophidian has hit the nail of the head. The performance results (primarily sensitivity) from the LFT results that a lot of investors are nervous about and believe are a significant risk are based on a lab trials on known samples. The results will look very impressive and beat most expectations.
Again we are hearing that the ‘gold standard’ PCR tests can be on 70% accurate in the real world due to swabbing user issues.
Avacta will take off when this first performance data is released.
Damn - you've all got e at it now - that last Selectivity should read Specificty Doh !
one other point @GG - I don't think that the affimer test would give rise to a large number of false positive in the population due to the highly selective nature of the affimer ****tail. Basically - if the test said you have it - then you pretty much can be sure you have it. - Not so certain of course if it doesn't as you could be early in the incubation and below the LOD.
This will not in my view give rise to large numbers of people self isolating for no reason.
Ophidian
@Genegenie - you are both right and wrong. FDA guidelines actually say:
" As such, FDA now recommends that developers use positive clinical samples for clinical validation. Moreover,
due to the increased availability of clinical samples, FDA recommends that developers confirm
performance of their assay by testing a minimum of 30 positive specimens and 30 negative
specimens as determined by an authorized assay"
So it is not a case of the real world where patients might be infected but not detectable or where they might not be infected.
The establishment of those 90% and 95% limits are against known and verified samples. For the affimer test - I'd be very surprised if both the Clinical Sensitivity and Selectivity weren't 100% against those test criteria.
Ophidian
@WolfieBill FDA guidance on COVID19 tests: "Tests should accurately identify at least 90% of positive cases (sensitivity) and 95% of negative cases (specificity)".
These just aren't reasonable parameters for whole population screening. On a scale of 65 million tests (UK only) these figures would authorise a test that could find 3.25 million false positives
I don't really understand what you're trying to say in your last sentence, but from a marketing standpoint a company will sell a test on the basis of its quality, not just that it's been approved (assuming that your audience knows the difference). If a test is marketed direct to consumers, most uninformed people will assume that the result is completely accurate. If you're selling to a Government, or a huge corporation, there is due diligence involved, and often a tender process which really scrutinises the contract. Selling a test that doesn't perform as well as other available tests means you will have no large customers.
GG re "And yes, FDA approval numbers are a low bar, but it's a competitive market. Having FDA approval means Avacta/Cytiva can sell a test, it doesn't mean anyone would buy it"
What's your reasoning for stating that "FDA approval numbers are a low bar"? And would be a high(er) bar?
I'm not sure, myself, that purely from an approval point of view, that have something other than FDA approval would be an influence of buying / selling this product.
That's what she said
Chengdo4 evidence of what? Do you think I have access to the in-house results from Avacta relating to their IP, or perhaps early access to the next RNS? I'm waiting just like you.
If it's broad strokes about what defines sensitivity/specificity/PPVs, pick up a stats 101 text book. Or if you've got an hour to kill maybe try geekymedics .com/sensitivity-specificity-ppv-and-npv/