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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'

#41

Post by anaphor »

nn35 wrote: post #34
anaphor wrote: post #31

Can someone clarify which type of test they analyzed? 80% seems way too high. I would expect something closer to 10% at most (which would still mean the probability of a true positive might be very low per Bayes' theorem)

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.)

I wasn't implying that 80% of all tests were false positives. I'm talking about P(You don't have coronavirus | You test positive). 80% is far beyond what would be normal for these types of medical diagnostic tests. I would expect something like 1% or less for a really good test, and around 10% for a bad one, but I don't know if the test the Chinese were using is really bad in some way, which is why I'm asking for clarification there.

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

#42

Post by ineedasername »

Scarblac wrote: post #15
bluGill wrote: post #7

Earlier quoted context omitted.

I'm starting to hear stories that the Chinese people demanded more quarantine than was needed. The problem got bad enough that the government decided to comply as it wouldn't hurt much more than the amount of quarantine was needed and might even help.

I really wonder how we're going to look back at this in a few years. On the one hand, no matter what measures politics takes here (the Netherlands), every political party is clamouring for more. That suggests to me that we'll probably end up doing too much. On the other hand, the scenes from Hubei and Italy are horrific, we're going to get them here as well, it could be far worse, we absolutely must act now. On the t…

The thing is, if all these measures stop the spread, even if they were all justified, the outcomes are pretty much indistinguishable from an over reaction: both accurate response and over reaction have similar outcomes.

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

#43

Post by rossdavidh »

nn35 wrote: post #34
anaphor wrote: post #31

Can someone clarify which type of test they analyzed? 80% seems way too high. I would expect something closer to 10% at most (which would still mean the probability of a true positive might be very low per Bayes' theorem)

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.

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

#44

Post by 9HZZRfNlpR »

Cerium wrote: post #23
ses1984 wrote: post #19

Earlier quoted context omitted.

>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. Can you elaborate on this a little more...?

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.

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

#45

Post by hanche »

andrewseanryan wrote: post #29
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…

Would I be correct with the following: If the false positive rate is higher than the expected rate of disease in a given community, then the majority of positive tests will be false positives. Does this relate to COVID in any way? Since the rates among affected communities seem to be growing rapidly. Would appreciate your thoughts.

If the false positive rate is p and the false negative rate is q, and the infection rate is r, then you will have p·(1-r) false positives (as proportion of the tested population) and (1-q)·r true positives. Your hypothesis p>r is not enough to settle which of those two numbers is bigger.

(Edited to fix a silly mistake: The phone rang while I was posting, so I ended up being hasty.)

Edit the 2nd: Even in the simplified case q=0, you can't easily tell.

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

#46

Post by usrusr »

andrewseanryan wrote: post #29
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…

Would I be correct with the following: If the false positive rate is higher than the expected rate of disease in a given community, then the majority of positive tests will be false positives. Does this relate to COVID in any way? Since the rates among affected communities seem to be growing rapidly. Would appreciate your thoughts.

Looking at growth rate with false positives is a bit of a mindbender: if you limit your testing to the potential contacts of a positive (false or not), you could get a "false R0" virtual epidemic from testing alone, if and only if you test more contacts per positive than 1/false positive rate. Unfortunately, actual hospitalizations and and deaths rule out a virtual epidemic so this is not a hope to cling to.

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

#48

Post by cs702 »

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.

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

#50

Post by [deleted] »

nn35 wrote: post #34
anaphor wrote: post #31

Can someone clarify which type of test they analyzed? 80% seems way too high. I would expect something closer to 10% at most (which would still mean the probability of a true positive might be very low per Bayes' theorem)

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.)

[deleted]
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