FAQ: Why is a lab test w a 12-48hr delay not as useful for screening?
Simple
Even super high sensitivity lab tests have 0% sensitivity while awaiting results
So you can be @ peak viral load & superspreading virus and not be detected by a lab test while spreading for 2 days
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For evaluating a tests “Effective sensitivity” to help stop transmission, this 0% sensitivity while awaiting results must be taken into account since ppl live their lives while waiting on results
If a test takes 2 days to return it’s effectiveness is already reduced by 33%
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So w a 2 day delay, a theoretical 100% sensitivity PCR test may already be maxing out at 67% effective sensitivity to slow spread... which is what we are trying to do w screening test programs.
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It is this “effective sensitivity” we should be measuring - not analytical sensitivity
Worse, unlike a rapid test that will detect just about anyone at peak titers, the loss in sensitivity due to waiting on a lab test can occur when someone is at peak virus transmission.
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So the two reasons for a loss in sensitivity between a rapid test and a lab test are not equal. A rapid test will lose sensitivity when virus is low and ppl are no longer infectious.
A Lab test will lose sensitivity regardless of being at super high virus load.
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This distinction in loss of effective sensitivity hasn’t yet been widely discussed but is extremely important.
A test that is rapid and misses those who are barely or not at all infectious may not be great for medical diagnostics but may be ideal for public health...
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And likewise a slower lab based Test may be great for medical diagnostics but not always ideal for public health.
They each have their (not mutually exclusive) places.
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Surprised at lack of trust the government has in the Irish ppl
Instead of supporting access to simple at-home tests to tell if u r infectious, position appears to be to limit access bc they don't trust ppl to handle a negative test
We do not defeat a pandemic without properly engaging & trusting in the public
To actively advocate against an asymptomatic person to be able to access a rapid test that has a very high sensitivity to tell you if you are spreading the virus doesn't make sense.
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Similarly to actively want to force symptomatic ppl to NOT have access to at-home tests bc u want ensure they get a much less accessible PCR test isn't good policy. Do we want symptomatic ppl leaving home to get PCR tests?? A rapid test is accessible and fast.
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This is also why lower analytical sensitivity tests, when performed frequently, are MUCH more sensitive than very high sensitivity tests performed infrequently or w slow return of results
If infectious, you will be detected by rapid Ag tests. And that’s what matters.
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But if you aren’t testing frequently, you are VERY likely to miss getting a swab in your nose before or when you are at peak infectivity (bc peak is reached VERY fast and diminishes fast).
So a test that takes a long time to return or is infrequent, simply doesn’t help much.
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Quarantines take a major toll on individuals. Physical and emotional. Most contacts in quarantine never become positive and there are multiple contacts for each positive.
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A quarantine of an uninfected individual should be considered with the same concern as false positives - and false positives sure have gotten a LOT of attention. But unlike a false positive, you currently can’t really get a “confirmatory test” to help get out of quarantine.
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To: Scientists, Doctors and anyone writing OpEds or Making Policy Decisions about tests
PLEASE NOTE:
The number of False Positive tests does *NOT* go up when prevalence of #COVID19 goes down.
The number of False Positives is the SAME regardless of high vs low prevalence.
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This idea keeps getting thrown around in OpEds and by decision makers - worried false positives sky rocket once disease drops.
But if you have 10 False Pos per 100,000 tests when incidence is high, you’ll have ~10 False Positives per 100,000 tests when incidence is low.
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What changes is not the number of false pos results, but the fraction of all pos that may be false
Of course even if false pos rate is 0.0001 (about that of newer rapid Ag tests), if there is zero Covid, then sure, 100% of positives will be false. But it’s still only 1 in 10,000
UK ppl should be concerned that head of @RoyalStatSoc#COVID19 diagnostics grp is inaccurately stating facts about testing-Intentionally conflating false positives w positive predictive value (totally different) to make rapid tests look bad #badscience 1/
In the tweet above, to get false positive rate you need to have the number of true negatives in the denominator. If you fail to do that, you aren't measuring the false positive rate. It's a totally different number. Studies show false positive rates for many tests <0.1%.
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In the first tweet I said "UK ppl should be concerned"... but c'mon @RoyalStatSoc - why allow this to continue. He's been stating falsehoods and bending stats or performing flagrantly unacceptable/unpublishable analyses for months now. Sowing confusion. How is this helpful?