I think that even at a certain point the risk of AIDS (I assume this is the reason) is better than death; so they should let in everyone letting both parties know the risks and say, we can wait longer or you can take this chance.
I'm no doctor/expert so the following is questioning not answering: is there any way to know an exact cutoff, i.e. "if we don't do it this second all is lost". I would assume not.
If my assumption is correct, then would you rather take a possibly HIV-infected donation now, that, if it turns out to be clean, gives you x% chance of living, or wait and see if a little more time brings you a donation that is definitely clean, even if the waiting gives you y% chance of living (where y is lower than x). Without knowing what x and y are, and without knowing the probability that the possibly-infected donation is infected, and without knowing how long you might have to wait to get a definitely clean donation... how do you make the call.
