In Brazil, where health system sucks, by the way, every person with a driver's license has the blood type printed on it. I know my blood type since I was a little kid, I was -*test*-('")ed at school.Japanese posters have said the same thing on other sites, but North America isn't like that. Like blue, I only found out when I donated blood - I still wouldn't know if I hadn't, and lots of people never donate blood. And while I'm no medical expert I would be very surprised if blood typing were done as part of regular checkups.
Well certainly the point is that Wilson thinks its strange that House knows. Enough to make an issue out of it. So it can't be some sort of common knowledge or a discussion they had, House sort of tried to explain it away like that and Wilson didn't go for it.I agree. Whether it's common to know your blood type in whatever country you live in doesn't matter - in the context of the show it was presented to the viewer, through Wilson's reaction, as significant. For that reason, I have to disagree with most of the interpretations: that it was on a file, House knew it from some blood drive, etc. Wilson clearly stated he'd never told House and his surprise at House's knowledge leaves out those other occasions. He knows House has gone through his personnel file (which wouldn't have that info anyway), if it had come up in some other situation Wilson wouldn't have been surprised.
House and Wilson are in the medical field, and I'd be surprised if this little fact didn't come up at some point in conversation (even though I could see House obtaining that knowledge in other ways).I believe Wilson specifically said, "I never told you my blood type" and he really pushed the question of how House knew, which House avoided. So like I said, if it was something that casually came up or was common knowledge between then Wilson would have known and not made it a big deal.