as far as i know, they do a finger prick to test you Hb. They do a screening questionnaire about "risky" behaviour so for example if you've ever recieved blood products before, ever had sex with a man who's had sex with men, ever been a prostitute, or slept with epople from certian countries. I also think they ask about tattoos and piercings. I think that's about all the screening they do before you donate
they test the blood that goes off too. But DO NOT ever use the blood donating service as a test for any of the diseases covered, Hep or aids as some blood can still get through tests even if infected.
i was shocked to find out that when my friend went to donate blood recently she was turned down because she was suffering with hayfever that day. I know i get turned away because im aneamic.
You also can't donate if you've had a transfusion since 1980 because of the (very small) risk of having vCJD. I don't agree with this one either - I didn't eat meat for nearly 20 years so was probably at a much lower risk than someone who ate burgers every day, but because I had a transfusion after my daughter was born I'm barred from donating.