It wouldn't be directly shared with your employer. But if everyone is there and you decline to participate in giving blood, it can point to information about you that you don't want people to know.
Whether you did or did not have some procedure done is definitely private medical information in Europe. There is no way for them to know whether I did or did not participate - maybe through the on site doctor if that is the one doing the procedure, but they're bound by very serious regulation to keep their mouths shut. Regardless, me not wanting to go would be a perfectly normal thing as well.
Blood donation tends to not be treated in the US as a medical procedure. It tends to be treated as a feel good community event. I'm a bit weirded out to have that reflected back to me as a medical procedure, though it certainly is. We don't quite seem to get that fact in some important way.
We're basically savages in huts over here about some things.
When I had a corporate job, lower level employees were instructed to keep their mouths shut and not tell everyone they were being promoted or whatever until it could be officially announced. Meanwhile, it was common for more than one middle manager type to drop by their cubicle to loudly congratulate them and make small talk, clearly trying to get in good with someone whose skills and such they might need.
I guess we were all supposed to be stupid or something and be incapable of inferring they had been promoted or something.
This was at a Fortune 500 company, so "the best of the best, sir." And it drove me crazy for so many reasons.
My mother is a German immigrant who came from a family of twelve kids. I am routinely shocked and appalled at how bad so many people are at thinking about the larger social landscape and how this will be viewed by others and what knock on effects it may have.
That type of thing seems to be shockingly common in the US and probably plays a large role in a lot of our social issues.
