[magnus]

So I'm a little confused with these false negative numbers.   In a different thread,  MT pointed out how PCR tests aren't close to 100% on negatives even with symptomatic cases.   I also saw somewhere that depending on where you got the sample from a person, the chances of a representative sample varied. 



So is the Abbott 16% false negative on top of the say 70% hit rate for a nasal swab?  In other words,  using abbott's quick test on nasal swabs would yield over 40% false negatives?



Yikes!  

Makes you wonder about all those hospitals relaxing PPE requirements on negative resulted patients.

I don't think so.  In an ideal world, the sensitivity and specificity of a test is determined by comparing it to a "gold standard" that is 100% or nearly 100% accurate.  In the real world, since we don't know how good the "gold standard" is, the measures of sensitivity and specificity are determined independently in a artificially constructed cohort.  It is possible to get samples that you know for certain are negative, without having to rely on a gold standard test to rule them out for disease; you pull stored samples from before the pandemic began.  Nobody should have antibodies for SARS-CoV-2 before November 2019, so you can be sure that those are true negatives.  Any positive serology result in a sample from pre-November-2019 can be considered a false positive.

For known positives, you can be fairly sure that anyone who had a positive viral RNA test was a true positive.  A false positive viral PCR test would be exceedingly rare (it would have to involve something like a sample mix-up or contamination, as PCR is extremely specific), so you can be fairly sure that anyone who tested positive for COVID-19 with conventional tests are true positives.

But yes, otherwise, if you are comparing a test to an imperfect gold standard, there are some issues, and that can be adjusted for to some extent.

BC

Sorry to request a further explanation, but I'm not sure what your answer is to my question about what the true false negative rating is for say a nasal swab being tested by 84% accurate (16% false negative) abbott's fast test. (the comments about false positives, I'm not disputing)

In other words, if you nasal swabbed and abbott tested 100 positive patients, how many results would end up positive (statistically).  At least 16% for sure, but would it be 40% if nasal swabs are only 70% capable of grabbing enough sample to trigger a positive result?

Thanks

[magnus]

So I'm a little confused with these false negative numbers.   In a different thread,  MT pointed out how PCR tests aren't close to 100% on negatives even with symptomatic cases.   I also saw somewhere that depending on where you got the sample from a person, the chances of a representative sample varied. 



So is the Abbott 16% false negative on top of the say 70% hit rate for a nasal swab?  In other words,  using abbott's quick test on nasal swabs would yield over 40% false negatives?



Yikes!  

Makes you wonder about all those hospitals relaxing PPE requirements on negative resulted patients.

I don't think so.  In an ideal world, the sensitivity and specificity of a test is determined by comparing it to a "gold standard" that is 100% or nearly 100% accurate.  In the real world, since we don't know how good the "gold standard" is, the measures of sensitivity and specificity are determined independently in a artificially constructed cohort.  It is possible to get samples that you know for certain are negative, without having to rely on a gold standard test to rule them out for disease; you pull stored samples from before the pandemic began.  Nobody should have antibodies for SARS-CoV-2 before November 2019, so you can be sure that those are true negatives.  Any positive serology result in a sample from pre-November-2019 can be considered a false positive.

For known positives, you can be fairly sure that anyone who had a positive viral RNA test was a true positive.  A false positive viral PCR test would be exceedingly rare (it would have to involve something like a sample mix-up or contamination, as PCR is extremely specific), so you can be fairly sure that anyone who tested positive for COVID-19 with conventional tests are true positives.

But yes, otherwise, if you are comparing a test to an imperfect gold standard, there are some issues, and that can be adjusted for to some extent.

BC

Sorry to request a further explanation, but I'm not sure what your answer is to my question about what the true false negative rating is for say a nasal swab being tested by 84% accurate (16% false negative) abbott's fast test. (the comments about false positives, I'm not disputing)

In other words, if you nasal swabbed and abbott tested 100 positive patients, how many results would end up positive (statistically).  At least 16% for sure, but would it be 40% if nasal swabs are only 70% capable of grabbing enough sample to trigger a positive result?

Thanks