I don;t know, but they do say not to turn up to a session if you think you need an hiv test.
My medical history doesn't change in the 16 weeks between donations, but I have to fill in the same questionnaire everytime, I even have to declare if I have had a paracetamol during the week before I am due to donate.
I suppose it would be a waste of time and resources if they ended up throwing away infected blood.
Also, apparently, only 6% of potential donors bother to donate, this would put the high risk groups firmly in the minority.