If the blood has THC in it, it probably shouldn't be donated because the person receiving the blood may be taking other drugs that could have adverse reactions to THC. But to answer the question, they test for a lot of things, even things that may surprise you. I know I can't donate blood since they determined that my immune system will go gangsta against anything that isn't me. The good news is that I now know why I didn't get as sick as many people in Boot Camp and don't get as sick as other people when exposed to the flu. The bad news is that I'm one of the few who is well enough to do extended guard duty during the season. The worst news is that my liver could be threatened by my own immune system, but nothing has come of it yet, here's hoping.
I miss my free Nutter Butters and apple juice man...