Can anyone with a scientific background explain why this invalidates test results?

[MrsMigginsMate]

I don't understand why the test is invalidated if you accidentally touch your tongue or teeth while swabbing? Or if your unclean fingers touch the lower part of the swab stick?

If its because your tongue could wipe off the virus then surely the rough surface of the swab would still retain some both in the grooves and also on the sides that didn't wipe against your tongue. And if you have covid on your fingers that's a pretty safe bet that you're infected (or about to be). So from a non scientific perspective it seems a bit of an overreaction to start again with a new swab. I'm sure there are good reasons, just wondering if someone can explain?

All instructions intended to be read and followed by a wide audience need to be as restrictive as possible, erring on the side of extreme caution. The story about someone drying their cat in the microwave and then suing the manufacturer for unclear instructions is probably an urban myth, but everyone who has ever worked with any sort of scientific surveys / data gathering will confirm it is not too unrealistic .

There are technical contamination issues with the PCR processes (even small amounts of foreign dna/rna can amplify massively during the process), but you are right - accidentally lightly touching your own clean teeth while taking the swab is unlikely to result in problems.

So, I think the rationale here is more managerial rather than scientific. Otherwise labs would be getting swab sticks covered in toothpaste, dropped in the morning coffee etc.

I'm interested in this as I did the test this morning and found it hard to keep my tongue down to do the swab.

You are trying to collect cells, not saliva, so possibly that is why they are so keen on emphasising the swab not being of teeth or tongue.

I used to work in a lab doing genetic screening for dogs by cheek swab in a similar way and the big problem there was owners getting the dog to open its mouth for the test by slipping it a treat to eat, so the main RNA on the swab ended up being cow/sheep/pig not dog...