It's best to have clean hands for no contaminants can be drawn up when taking a sample of blood. It does not take much to throw the test. This is my understanding.
There are times when I haven't washed and the number is believable and sometimes the numbers are way high... with the latter the numbers often do come down after having washed hands...
It's best to take it off the table.
you can actually have touched something that changes the blood-tests result, you can also with your needle inject bacteria directly into your body which can lead to infections... like if you have touched something with salmonelle bacteria... or streptococcus
but usually nothing happens because the needle stitch is not that deep
If you have something sweet on your hands like fruit juice if you have been eating fruit, or sugar or chocolate, even toothpaste it could affect the reading.
If you have something sweet on your hands like fruit juice if you have been eating fruit, or sugar or chocolate, even toothpaste it could affect the reading.
Or apparently some soaps can contain sugars! And I used to use a hand cream that contained some sweet tasting ingredients and had to make quite sure there was none of it left on my hands before testing.