If you're in the US, they did an anemia check before the donation and your iron level was high enough to safely donate. I'm anemic and struggle to get into the safe range but I've never passed out in all my years of donating even when I barely passed. More likely the cause of you passing out was a sudden drop in blood pressure. My sister has low blood pressure normally and another friend is a very petite woman and they've both fainted on occasion after donating. They still donate though, they just rest afterwards before moving and have someone else drive them home. source: 5 gallon blood donor and 3 year blood bank volunteer.

So, I've donated blood twice. The first time I passed out, but being nerveous and not eating probably didn't help. The second time I went, I didn't pass out, but I did go pale and they kept checking my stats for a long while. First of all, you are NOT meant to feel dizzy AT ALL, maybe a tad weakish, but not actually dizzy. I said the same thing to the doctors how I thought it was normal and they said that that's a myth. Secondly, they measure your hemoglobin when donating. Mine has always been normal, if anything maybe a tad over hence I was cleared for donating. A few months later, I'd been feeling really tired, so I got some blood tests done, including ferritin which is what they actually use to diagnose aenemia (normal is between 10-150, mine was like 7). Basically, ferritin is the grapes and hemoglobin is the wine and usually these should correspond, but not always = aenemia. So basically you had enough iron to pump up your hemoglobin for now, but not enough to do it again. And that's a special test you need to order and is not done at the donation centre, cause in most cases, it's not necessary.

Did you at least get Orange juice?

THEY DIDN'T WARN ME THAT I WAS ANEMIC You deserve it

How were they supposed to know you were anemic? that's ridiculous.

At blood drives, the nurses typically take a sample of blood before the donation to check for things like anemia. If this is the case and they neglected to tell you (and it's possible they figured you'd be okay if you were just borderline) then that's a fault on their part. Passing out might be quite embarrassing, but at least you're okay! And if the anemia starts bothering you in daily life, you may want to consider taking iron supplements. Get better, OP!

They should have done a finger poke and tested your blood beforehand to see what your hemoglobin levels where, that's just a shoddy donation process in my opinion. Every time I've given blood they've checked my hemoglobin, otherwise it's just a waste of both parties time since they can't use that blood and could have spent the time drawing a more appropriate candidate.

Just so everyone knows, a basic finger prick can miss anaemia. Anaemia and low iron are not necessarily the same thing. Although I get the feeling OP probably has neither and simply passed out because s/he (sorry, I didn't take note) didn't go have a meal and rest, they went shopping. Passing out rarely takes you by surprise either. You generally at least have time to safely sit straight down on the floor and arrange your arms so you don't hit your head if you go on to lose consciousness. Although sitting tends to fix it. At one point you're in the shop thinking "wow, I feel like garbage, I just had blood taken and I'm very light headed. I'm not going to go home and rest. I'm not even going to stop shopping and seek out a chair. I'm going to keep right on until I keel over". I can't feel sorry for you.

You sure "borderline anemia" made you pass out? I get VERY anemic (like iron transfusion anemic...) and I've never even gotten dizzy when they draw blood... be careful dude

what do you mean, they didn't warn you? how the he'll are they supposed to know you were anemic??