Anyone sciencey understand in detail PCRs?

[Tinysnickers]

Trying to settle an argument and google is not helping.
If you did a covid PCR swab but didn't actually swab yourself, just put the swab in the tube of liquid and sent it off, what result would you get?
A) inconclusive /void because nothing at all is detected?
B) negative because it has no covid on it.

Do the PCR results machines 'see' anything else eg human dna, other rhinovirus, or does it look only for covid?

Does anyone know?

Negative. PCR takes tiny amounts of dna and replicates it so there’s enough to use for a test (be that a paternity test, a crime scene sample or a viral detection test).

Inconclusive

The test result will be negative, PCR tests are very specific and will only tell you whether Covid is present or not.

It really depends on the assay they use. It can be negative if the lab tests for the presence of sars cov2 only or they also use a control. Control can be any gene expressed in human epithelial cells (this is what you are essentially swabbing off your nasal cavity). If they use the latter, results will be inconclusive essentially showing there was no human (and viral) RNA detected in the sample.
I do a lot of PCR for my job but personally not familiar with sars cov2 assay

Inconclusive. In PCR you always run a control for something so you know should show up so when it does not, you know there's a problem with the sample or the machine. I am fairly sure there's a control for some common element of human dna. If you don't swab at all, the control won't show up and the result will be inconclusive.

Negative.

The RT-PCR tests usually target 2 (or more) distinct gene regions of the virus. An inconclusive test is when only 1 of the 2 targets are found.

This is an easy to follow explanation of how the test works:
www.scientificamerican.com/video/the-science-behind-how-coronavirus-tests-work/

Negative.

‘Depends on whether there is an internal check for human tissue’according to dh (who does PCRs, although not for human covid. If they don’t have that internal check it would be negative, if they do (which would be best practice ;-)) it would be void. The amount of rna should be measured before the test is developed (which would tell you if no swab) but he doesn’t know if that would be done in this instance :-)

I was arguing for inconclusive /void because I was guessing there should be some sort of control to make sure something had been swabbed.
But on the other hand, after the immensa lab cockup, who knows!!

There must be a control, they couldn't certify you as fit to fly etc if you just waved the swab around and handed it back.
Even the lfts have a control.

Its negative

I know loads who've done this. Lateral flow are the same - those ive tested myself!

Most inconclusive tests are actually positive

Decay - try it on an lft. I guarantee if you don't touch your face in any way you will get a negative result - i flew over the summer and everyone who i met who flew with Tui so did lft via photo upload did this - stops you being caught abroad. Im not saying its right but you absolutely can fake negative tests.

Depends on the assay- some will have a control in it which detects human cells and can see if the sample is taken property. In which case it a repeat test would be requested

There must be a control, they couldn't certify you as fit to fly etc if you just waved the swab around and handed it back.
Even the lfts have a control

Yes, scientific tests like pregnancy tests and lateral flows have an internal control, but it works to ensure the liquid has migrated up the strip, not to ensure that someone had used the swab.

PCR controls are more involved, but generally show that the machine doing the test has worked as it should with a control molecule. I work in an NHS lab and we run PCR tests often, for all sorts of stuff.

Solution only gives a negative lft, Diet Coke gives a positive and white wine gives a T line but no C (I was bored)…

That’s why unsupervised lfts are utterly totally pointless to prove anything!

I did wonder about the pcr too…

Shit, I've been swabbing a screaming struggling DS and assuming I've caught enough snot because the control line showed up. Brilliant. Well, you live and learn...