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Why are PCR test primarily used for testing symptomatic #COVID19 people? 
Because it would produce too many 'false' positives if used for testing of asymptomatic #COVID19 people. What??? Yessee
1/x
PCR tests are based on detecting SARS-CoV-2 RNA from the upper respiratory tract.
Research suggest the mean duration of SARS-CoV-2 RNA shedding in URT is 17 days.
However, no study detected live virus beyond day 9 of illness.
2/x https://doi.org/10.1016/S2666-5247(20)30172-5
Research suggest the mean duration of SARS-CoV-2 RNA shedding in URT is 17 days.
However, no study detected live virus beyond day 9 of illness.
2/x https://doi.org/10.1016/S2666-5247(20)30172-5
This suggest, if you have a +ve PCR test shortly after you have developed symptoms it is VERY likely that you are infectious.
But, if you don't have symptoms and +ve
only 50:50 chance that you are still in the infectious time window.
3/x
But, if you don't have symptoms and +ve
only 50:50 chance that you are still in the infectious time window. 3/x
In other words - the risk of 'false' +ve PCR tests is high among asymptomatic people
By 'false' I mean people get +ve PCR result because they still have SARS-CoV-2 RNA in their body, but they are no longer infectious.
4/x
By 'false' I mean people get +ve PCR result because they still have SARS-CoV-2 RNA in their body, but they are no longer infectious.
4/x
So is Rapid Ag or lateral flow tests better for testing of asymptomatic people?YesWhat about the false negatives?Rapid Ag produce VERY FEW real false negatives5/x
Rapid Ag tests identify live SARS-CoV-2 material.
So if you test +ve it is VERY likely that you are infectious even if you have NO symptoms.
6/x
So if you test +ve it is VERY likely that you are infectious even if you have NO symptoms.
6/x
If you test -ve with a Rapid Ag test it means the test did not detect any live virus.
The chance that you are infectious is therefore very low.
7/x
The chance that you are infectious is therefore very low.
7/x
Some people have compared PCR and Rapid Ag tests and came to the conclusion that Rapid Ag had a low sensitivity ~ many false negatives.
However, they 'forgot' to take into account PCR tests are too sensitive and produce too many 'false' positives.
8/x
However, they 'forgot' to take into account PCR tests are too sensitive and produce too many 'false' positives.
8/x
So when you see a study saying the sensitivity of Rapid Ag test was 50 - 60% you should ask
compared to what?
If it is to a PCR test you will know the true sensitivity of the Rapid Ag is likely to high.
9/x
compared to what?
If it is to a PCR test you will know the true sensitivity of the Rapid Ag is likely to high.
9/x
So should we continue to use PCR tests to test symptomatic individuals? Yes
