
Potential false-positive rate among the 'asymptomatic infected individuals' - trampi
https://www.ncbi.nlm.nih.gov/pubmed/32133832
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cultus
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.

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ses1984
>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...?

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Cerium
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 positives.

~~~
andrewseanryan
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.

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usrusr
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.

~~~
AnthonyMouse
> Unfortunately, actual hospitalizations and and deaths rule out a virtual
> epidemic so this is not a hope to cling to.

Not necessarily. In theory all the deaths could have some other cause, i.e.
some fraction of people with a different underlying fatal condition had false
positive tests for this coronavirus and then died of the other condition.

That's probably not what's happening, but it's theoretically possible. (It's
also probable that _some_ of the reported deaths _are_ that, but who knows
what percentage.)

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lbj
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.

~~~
jmartinpetersen
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 - still
go shopping for clothes, gardening stuff, electronics and most importantly
food.

Second, although the right to forced entry into private property was in the
original draft it was removed before vote. Entry still follows the known rules
of needing approval by a judge.

You are right, however, that forced testing, forced treatment, forced
vaccination (if/when possible) and forced quarantine is mandated as per
discretion of the public health authorities.

~~~
hanche
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.

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wycy
Would this indicate that the mortality rate is actually much higher
percentage-wise, since the denominator is actually artificially inflated?

~~~
wjnc
Not that I'd expect, since non-testing in probable positive cases (f.e. in NL
those sick, but manageable and in home quarantine are usually not tested)
seems to dwarf false-positives in negative cases.

~~~
coding123
But could that just be the flu.

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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...](https://arstechnica.com/science/2020/03/dont-panic-the-
comprehensive-ars-technica-guide-to-the-coronavirus/2/)

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nn35
How did they estimate this? If anybody can read the actual paper, I’d love to
know.

If false positives dominate true positives then you’d expect total positives
to depend primarily on number of tests given, right? Which sounds wrong to me,
but I’d be interested in hearing other thoughts.

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anaphor
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)

~~~
nn35
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.)

~~~
rossdavidh
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.

~~~
dnautics
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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nck4222
Interesting, as this means that China would be quarantining more people than
"necessary", which would help slow the pandemic anyway. I can't imagine an
asymptomatic person would put stress on the hospital system? But maybe I'm
wrong there.

I am curious if this also could indicate a false-positive problem with non
asymptomatic people as well.

False-positives are also why the CDC tests had to be shipped back, although
that was because it was showing false positives in other diseases it was
testing for, not COVID-19.

~~~
gus_massa
> _I can 't imagine an asymptomatic person would put stress on the hospital
> system?_

If s/he is in the hospital in quarantine you must give blankets and food,
probably a nurse to check the temperature and symptoms two or three times per
day, a medical doctor one a day just to be sure. Perhaps a blood analysis from
time to time?

Luckily you don't have to handle visitors because they are in quarantine. (Or
there are some visits? What if one patient tries to escape?) You must give an
official reports for the family. Now you can assume the patient can send a
WhatsApp message to the family saying s/he is fine, but you need probably
still an official report. Paperwork, there is also paperwork.

How isolated are them from each other. If they are all together, you can
transform the overcrowded false positives in real patients.

~~~
gpderetta
why would an asymptomatic person be in an hospital? Even people with minor
symptoms are asked to self quarantine at home pretty much everywhere.

~~~
ineedasername
In China, asymptomatic with a positive test result still put you in their
make-shift hospital facilities.

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mempko
Wait, so the actual mortality rate for COVID-19 is much higher than thought
because of all the false positives on cases without symptoms?

~~~
taborj
But that also means that it doesn't spread as quickly/easily as originally
thought. Right?

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nknealk
Anyone save the full text? Every time I try to get to it I get a 404.

~~~
RobertDeNiro
I believe the full article is in Chinese.

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virusduck
It's not in anything now. It's 404'd which is concerning.

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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.

~~~
AnthonyMouse
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.

~~~
cs702
True. I updated my comment.

