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Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

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Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#51

Post by oefrha »

DeonPenny wrote: post #30
acchow wrote: post #27

Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random

Your never going to have a completely random test. Your randomness comes from you picking for an area and the people who chose to do the test. Saying otherwise is being pedantic to your detriment.

> Your never going to have a completely random test.

You are not, but on the full spectrum of somewhat random selection methods, talking to people on the street is about as biased as you can get.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#52

Post by fhsm »

acchow wrote: post #27

Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random

“Public health experts already knew Chelsea had the state’s highest rate of confirmed COVID-19 cases and that the actual rate was probably higher”

Should be 1/3 + in town with highest confirmed case rate. It may be a random sample within that town but it’s explicitly not representative of MA

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#53

Post by Rantenki »

Alright. So what's the rate of false positives for that test?

In other words, if this test (and similarly the stanford study in LA) have even a small rate of false positives, combined with any sampling bias, it's going to lead to some misleading conclusions.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#54

Post by adtechperson »

arenaninja wrote: post #33
acchow wrote: post #27

Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random

Yep.. I saw a similar headline claiming LA infections are 55x the reported rate. I think the math is likely simpler, and the infection is somewhere between 3x-5x (10x tops). I've read a few reports saying about 50% of people are asymptomatic, 30% show symptoms that aren't bad enough for hospitalization and 20% require hospitalization. I can't find the source I got it from but I believe it was a Chinese study

This also came out today. http://publichealth.lacounty.gov/phcommon/public/media/media... We really have no idea what the IFR is, just that is probably in the .1% to 1% range. Also, it takes around 2 weeks to develop antibodies, so the tests are showing how many people had covid 2 weeks before testing.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#55

Post by retsibsi »

fortran77 wrote: post #25
wtvanhest wrote: post #11

This is another irresponsible headline. I cannot read the article, so correct me if I am wrong, but if what others said is true... It was not random because they didn't randomly select people, they let people self-select! If they gave the total number of people who took the test, and total number of people asked, you could at least get a range. Of course, that range would still be biased to people who are walking aro…

This is a strong response from someone who claims not to have read the article.

Is it wrong, though? I did read the article, and I don't think so.

One relevant thing GP may have missed is that the researchers "excluded anyone who had tested positive for the virus in the standard nasal swab test". So they weren't quite trying to directly estimate the proportion of people with antibodies, but the proportion of people without a positive test with antibodies. But I suspect this didn't change the results much: how many people in the area have tested positive and recovered sufficiently to be out in public?

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#56

Post by animalCrax0rz »

acchow wrote: post #27

Can we change the title? Article title is "Nearly a third of 200 blood samples taken in Chelsea show exposure to coronavirus" The sample is clearly not random

In addition, the wording "test positive for Covid antibodies" may lead people to think that 30% of the population in MA is immune to Covid19. It obfuscates the fact that the mere presence of antibodies does not imply immunity. There has to be a sufficient amount of antibodies in order to successfully fight off reinfection (which could be worse than the initial infection) ...

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#57

Post by notforever »

subroutine wrote: post #46
DenisM wrote: post #28

Earlier quoted context omitted.

Yes, it's 10% false positive. But it's not "really low" in the sense that it's typical for antibody tests. Which is one reason not to do wide-spread testing.

Or you could flip that around and make testing so plentiful everyone is tested 3x (perhaps by independent labs), so the FP rate is more like... 1/10 * 1/10 * 1/10 (1/1000)

No good reason for test errors to be uncorrelated, ie. quite possible same people who trigger false positives on the first test would be false positives on the second etc.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#58

Post by zucker42 »

subroutine wrote: post #46
DenisM wrote: post #28

Earlier quoted context omitted.

Yes, it's 10% false positive. But it's not "really low" in the sense that it's typical for antibody tests. Which is one reason not to do wide-spread testing.

Or you could flip that around and make testing so plentiful everyone is tested 3x (perhaps by independent labs), so the FP rate is more like... 1/10 * 1/10 * 1/10 (1/1000)

Assuming false positives aren't correlated even with independently manufactured tests seems like a terrible idea to me, knowing little about medicine.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#59

Post by gmueckl »

subroutine wrote: post #46
DenisM wrote: post #28

Earlier quoted context omitted.

Yes, it's 10% false positive. But it's not "really low" in the sense that it's typical for antibody tests. Which is one reason not to do wide-spread testing.

Or you could flip that around and make testing so plentiful everyone is tested 3x (perhaps by independent labs), so the FP rate is more like... 1/10 * 1/10 * 1/10 (1/1000)

This assumes that the cause for false positive is in the test execution and not a contamination in the sample itself.

Re: Nearly a third of blood samples taken in Chelsea show exposure to coronavirus

#60

Post by jariel »

epmaybe wrote: post #21
Exmoor wrote: post #10

>Nearly one third of 200 Chelsea residents who gave a drop of blood to researchers on the street this week tested positive for antibodies linked to COVID-19, a startling indication of how widespread infections have been in the densely populated city. >The doctors used a diagnostic device made by BioMedomics, of Morrisville, N.C., to analyze drops of blood. It resembled an over-the-counter pregnancy test and generated…

Because you quoted the article using BioMedomics' antibody diagnostic test, I will quote the accuracy from their site[0]: > In order to test the detection sensitivity and specificity of the COVID-19 IgG-IgM combined antibody test, blood samples were collected from COVID-19 patients from multiple hospitals and Chinese CDC laboratories. The tests were done separately at each site. A total of 525 cases were tested: 397…

This specificity issue is bothering me.

This 'medical' meaning 'very important' - so if there's a test with 90% accuracy, why on heaven's earth do we not simply run the test 3 times to get 'considerably greater accuracy'?

10% error is so large it's tough to make heads or tails of the data?

Does someone know if this makes sense i.e. if we can simply run the test 3 times and get better data.

Secondly - why are these health authorities not doing proper, state-wide tests?

Here we are with an economy in meltdown, trying to 'model model model' with a 10 Trillion dollar economy wouldn't it make sense for at least ONE (or a few) freaking comprehensive tests that give us some good data?

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