I used to give blood many years ago, even though I nearly passed out the first 3 times!(Feet up on a box, cold flannel on the forehead and a nurse fanning me with a piece of cardboard

) Then after a few years I got a letter asking me not to donate anymore as their tests had shown false reactions for the last 3 donations. They assured me that upon further testing there was nothing wrong with me, but after a reaction they couldn't use my blood.
Fast forward about 8 or 10 years and I received a letter saying that they have new tests now so would I donate again, which I have. So far, so good!