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New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

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Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#21

Post by binarymax »

klmadfejno wrote: post #10
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

What's wrong with 10% of them being false positives? (also I think it's 9%)

Because with a false positive antibody test then you can still contract and be a carrier. This is a problem for the immunity passport proposals (which I think are a terrible idea anyway, but that’s beside the point)

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#23

Post by gwd »

wtvanhest wrote: post #7
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

Would you or someone else mind expanding on this thought a little? Why does 99.9% specificity mean that 10% will be false positives? {added: great answers below} well worth understanding this point. In short specificity measures the % of the population tested which had false positives, but doesn't give you the ratio of false positives to positives or the probability that a positive test means you actually have the an…

Let me give it a try. Suppose we have 100,000 people in a statistically representative town.

If 1% of people have had COVID-19, then that's 1000 people who have had it, and 99,000 people who haven't.

The test has a sensitivity of 100%, which means all 1000 people who've had it will test positive.

The test has a specificity of 99.9%, which means 98,901 of the 99,000 people who haven't had it will test negative; but that leaves 99 people who haven't had it, but test positive anyway.

That gives us 1099 people who look like they have immunity; but only 91% of those people are actually immune: 9% of the people are false positives.

If instead we have a specificity of 99%, then only 98,010 of the 99,000 people who haven't had it will test negative, leaving 990 people who haven't had it but test positive anyway.

That gives us 1990 people who look like they have immunity; but only 50% of them actually do -- the other 50% are false positives.

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#24

Post by service_bus »

klmadfejno wrote: post #10
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

What's wrong with 10% of them being false positives? (also I think it's 9%)

The person you are responding to is suggesting a scenario like the movie 'contagion' where people marked clean are given a wristband or pass of some kind.

In reality testing can be used as an effective tool regardless of whether or not people can be 'certified'

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#26

Post by Mvandenbergh »

wtvanhest wrote: post #7
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

Would you or someone else mind expanding on this thought a little? Why does 99.9% specificity mean that 10% will be false positives? {added: great answers below} well worth understanding this point. In short specificity measures the % of the population tested which had false positives, but doesn't give you the ratio of false positives to positives or the probability that a positive test means you actually have the an…

Let's say you have 1000 people, 1% or 10 have had the virus and are seropositive (have antibodies).

Of the 900 people who do not have ABs, 99.9% or 899.1 are correctly identified as not having them, 0.9 is identified incorrectly as having them when they actually do not.

Of the 10 who actually have antibodies, 100% are correctly identified.

So 10.9 are identified as having antibodies, in 0.9 person's case incorrectly which is about 10%.

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#27

Post by [deleted] »

wtvanhest wrote: post #7
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

Would you or someone else mind expanding on this thought a little? Why does 99.9% specificity mean that 10% will be false positives? {added: great answers below} well worth understanding this point. In short specificity measures the % of the population tested which had false positives, but doesn't give you the ratio of false positives to positives or the probability that a positive test means you actually have the an…

[deleted]

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#28

Post by jointpdf »

wtvanhest wrote: post #7
pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

Would you or someone else mind expanding on this thought a little? Why does 99.9% specificity mean that 10% will be false positives? {added: great answers below} well worth understanding this point. In short specificity measures the % of the population tested which had false positives, but doesn't give you the ratio of false positives to positives or the probability that a positive test means you actually have the an…

via Bayes Rule: “Assuming an underlying infection rate P(I), what’s the probability that a person is actually immune (=was infected), given that they test positive, i.e. P(I|+)?”:

  P(I|+) 
  = 
  P(+|I)*P(I) / P(+)
  = 
  Sens*P(I) / [Sens*P(I) + (1-Spec)*(1-P(I))]
  =
  .01 / (.01 + .001*.99)
This is exhibit A of the base rate fallacy (https://en.m.wikipedia.org/wiki/Base_rate_fallacy).

Re: New Abbott SARS-CoV-2 antibody test has 99.90% specificity and 100% sensitivity

#30

Post by mlyle »

pgcudahy wrote: post #6

There seems to be no analysis of whether these are neutralizing antibodies. The idea of using serology for immunity certificates or "golden tickets" is never going to go well. Even with 99.9% specificity, if the population prevalence is 1%, 10% of positives will be false positives. If in real world testing, specificity is 99% and population prevalence is 1%, then 50% of positives are false positives.

> if the population prevalence is 1%

But the population prevalence is much more than 1%: 80k deaths at a 1% infection fatality rate (and I believe this is high, but I'm being conservative) implies 8,000,000 infections so far. This is more like 2.5%. So far. 95% CI for specificity is 99.5%, so you can be reasonably confident that you're doing better than 85%.

It may not be a perfect intervention, but you could really reduce risk. If there's an 85% chance that someone is immune, they do not share a household with a vulnerable person, and they are not in a high risk group themselves-- you've reduced the risk of death to basically nothing.

I disagree with it for other reasons (it incents people to go get sick to be free/be able to work/etc).

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