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Potential false-positive rate among the 'asymptomatic infected individuals'

ncbi.nlm.nih.gov

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Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#51

Post by hanche »

jmartinpetersen wrote: post #16
lbj wrote: post #9

In Denmark we increased our testing 10-fold and found 300% more people infected. Our response to that increase has been to shut down the country completely for 2 weeks and expand our governments right to act: Forced entry into private property, forced isolation and treatment, forced testing. If this is all because of an error in the test kit I'm going to be super ticked off.

This is somewhat misleading information. First, the country has NOT been completely shut down. I went shopping today and bought milk, yeast and flour. We didn't need toilet paper, but the store had plenty. All schools and most of the public sector closes down for two weeks on Monday. Some business (like restaurants, movie theaters and fitness gyms) are closing down on their own accord. But you can - if you will - sti…

The situation is similar in Norway. We're a bit more strict, as parts of the private sector is also forcibly shut down: Gyms, pubs, hairdressers, movie theaters are all closed.

Our infection rate has grown dramatically in the past few days, and not as a result of increased testing AFAIK. Testing capacity has been limited, but is being drastically increased as of today. So maybe the already high growth rate will increase further as a result.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#52

Post by AnthonyMouse »

cs702 wrote: post #48

If tests indeed have such a high false-positive rate, then all estimates of fatality rate calculated by dividing over the number of individuals identified as "infected" are too low, i.e., by implication the virus is actually deadlier than naively estimated. EDIT: All else remaining the same. See AnthonyMouse's comment below for important clarifications and corrections.

That's assuming a large fraction of the people who have been tested are asymptomatic, otherwise a high false positive rate among asymptomatic people would have minimal effect on the numbers because they aren't being tested to begin with.

Meanwhile you also have the opposite happening for the same reason -- if even a small percentage of asymptomatic people are actually infected but not being tested, a small percentage of "asymptomatic people" (i.e. nearly the entire population) could represent a very large proportion of those infected and cause the fatality rate estimates to be much higher than the true number.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#53

Post by andrewseanryan »

One big question is what percentage of positive tests are asymptomatic? If only a tiny percent are asymptomatic, then this false positive issue would not be elevating the total numbers much, right? I won’t claim to have the best resource here but one article stated:

“ Dr. Tedros noted that only 1 percent of cases in China are reported as “asymptomatic.” And of that 1 percent, 75 percent do go on to develop symptoms.”

https://arstechnica.com/science/2020/03/dont-panic-the-compr...

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#54

Post by AnthonyMouse »

9HZZRfNlpR wrote: post #44
Cerium wrote: post #23

Earlier quoted context omitted.

If a given test has a 1% chance of returning true, even when the actual result is false, then from a sample of say 1000 tests we would expect at least 10 trues, in addition to any actual true results. If the chance of having the disease in the general population is low (say 1 in a thousand for this example) then we would expect 11 true results in our thousand samples. Of which 91% are incorrect results - false positi…

How does it relate to universal Healthcare? I'm not an American so maybe I'm missing something.

A "fallacy that seems universal with healthcare folks" is not expected to be related to universal healthcare.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#55

Post by cs702 »

AnthonyMouse wrote: post #52
cs702 wrote: post #48

If tests indeed have such a high false-positive rate, then all estimates of fatality rate calculated by dividing over the number of individuals identified as "infected" are too low, i.e., by implication the virus is actually deadlier than naively estimated. EDIT: All else remaining the same. See AnthonyMouse's comment below for important clarifications and corrections.

That's assuming a large fraction of the people who have been tested are asymptomatic, otherwise a high false positive rate among asymptomatic people would have minimal effect on the numbers because they aren't being tested to begin with. Meanwhile you also have the opposite happening for the same reason -- if even a small percentage of asymptomatic people are actually infected but not being tested, a small percentage…

True. I updated my comment.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#56

Post by astrophysician »

mikorym wrote: post #36
cultus wrote: post #18

One fallacy that seems universal with healthcare folks is they think the false positive rate is the chance that a given positive result is erroneous. If an illness is rare, a positive result in a test with a 1% error rate might have an overwhelming probability of being a false positive. This is why prior probabilities need to be taken into account in making decisions.

I appreciate all the answers that point out Bayes's theorem. One thing to add is that in contrast to the usual classroom Bayes the "occurrence in the general population" is very much a variable rather than a constant. For example, Germany's low death rate can be a side effect of the false positive issue in the paper. In fact, the stark difference between Italy and Germany would provide support to the paper's conclusi…

Right, so it's a variable, but if we can independently nail down a false negative/positive rate, then we could try to infer the true rate of infection, but problem is (assuming US numbers):

* assume false positive rate of 10%, assume true positive rate of 100% (it's not but let's be generous)

* maybe 1/10 of the population (30 million) gets tested

* lets say 100,000 people have COVID right now (50x the official number)

number of positive tests = 30 million * 0.1 + 100k * 1 = 3,100,000

fraction of postive tests that actually have the disease = 1 / 31 = 3.2%

problem is FPR (1) depends on the population tested (i.e. p(covid | positive test) != p(covid|positive test, some symptoms) != ...), (2) we need very accurate measurements of FPR because:

lets say we constrain FPR to 10% +/- 1% ==> 10% uncertainty in FPR -- that means our inference of the number of infected people is:

n_infected = (n_positive_tests - FPR * n_tested)

which is: -200,000 to 400,000

so...not very useful.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#57

Post by dnautics »

ineedasername wrote: post #37
glofish wrote: post #14

Earlier quoted context omitted.

Mortality is almost always much lower than the initially reported and observed numbers. There could be many more people that never even get to the testing phase.

The same would be true of things like the Flu then too. Which means the true mortality rate of Flu is much lower, so even if COVID-19 is lower than current estimates for the same reason, it is still an order of magnitude higher than something like the Flu.

Yes, and that's why absolute population normalized mortality rate is the only really comparable thing across populations.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#58

Post by AnthonyMouse »

ineedasername wrote: post #37
glofish wrote: post #14

Earlier quoted context omitted.

Mortality is almost always much lower than the initially reported and observed numbers. There could be many more people that never even get to the testing phase.

The same would be true of things like the Flu then too. Which means the true mortality rate of Flu is much lower, so even if COVID-19 is lower than current estimates for the same reason, it is still an order of magnitude higher than something like the Flu.

It's possible to get more accurate numbers for the flu because it's so prevalent, which makes it possible to do random sampling of the general population. If you did random sampling of the general population for this coronavirus right now, ~100% of the positive results would be erroneous because probably less than 0.01% of the population actually has it.

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#59

Post by SkyPuncher »

cultus wrote: post #18

One fallacy that seems universal with healthcare folks is they think the false positive rate is the chance that a given positive result is erroneous. If an illness is rare, a positive result in a test with a 1% error rate might have an overwhelming probability of being a false positive. This is why prior probabilities need to be taken into account in making decisions.

With all due respect, this comment shows a lack of understanding about how health professionals assess the quality of diagnostics.

My wife is a doctor (and I've learned a lot from her). In med school, they're specifically taught to evaluate diagnostics on their specificity and sensitivity - which essentially covers false positives and false negatives. If you hear a doctor talk about the "accuracy" of a test, it's likely because they're simplifying the concepts.

"Error rate" or "accuracy" is not used at the scientific level in medicine. Partly, for the reason you defined. It doesn't convey enough information about the outcome of the test.

A "99% accurate test" is pretty meaningless without understanding the specificity and/or sensitivity components. In fact, I've seen some headlines where they incorrectly refer to only one component as the "accuracy".

Re: Potential false-positive rate among the 'asymptomatic infected individuals'

#60

Post by dnautics »

rossdavidh wrote: post #43
nn35 wrote: post #34

Earlier quoted context omitted.

Are you confused or am I misreading your comment? The result is that 80% of positives are false positives, not that 80% of all tests are false positives. (IMO it is still fishy.)

80%...in asymptomatic cases. So if most of the people who get tested DID show symptoms, the false positive rate generally could be far worse. But, it would suggest downsides to more general testing.

But if your diagnostic criteria is showing symptoms later, then you are ejecting the entire population of carriers who might be, say, teeming with the virus but showing zero symptoms, for perhaps a genetic or "dumb luck" reason.
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