Do not use the blood collections service as an HIV testing centre. If you are registered and have had a risk it is better you take yourself off the register for the pre-requisite time.
I'm sure you know, obviously, not to use that as a way to find out, where I donate blood they make you sign a form saying that isn't why you are doing it, anywho....I'm pretty sure it's damn accurate, I would assume that they hold the blood for a certain amount of time as well before they test it.