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???

Yes

see
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

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

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

So is Rapid Ag or lateral flow tests better for testing of asymptomatic people?

Yes

What about the false negatives?

Rapid Ag produce VERY FEW real false negatives

5/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

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

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

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

So should we continue to use PCR tests to test symptomatic individuals?

Yes