testing-possiblity of false positive?

I am a nurse that routinely conducts STD exams and just got through drawing blood for a confirmatory test on a patient that was told he/she had tested positive on a test at the local plasma donation center.  The patient brought in a form letter from this center which had several selections with a check mark beside HIV 1/2.  I called the plasma center and inquired as to what HIV test they run on donors.  The person who I talked to at the plasma center was supposed to call me back, but did not.  My thought is that the test was probably a preliminary screening test and that there is at least a slight possibility that this person could have a false positive.  What are the chances of this happening, in your experience?  Is anyone familiar with plasma donation centers and their policies?  It seems rather cold-blooded to just dump this on someone with a form letter, not to mention that the wrong box might have been checked.

[Teak]

Usually they get a rejection notice and to seek further follow ups with the doctor. They will then be retested and have to take a copy of their test result the next time they attempt to donate. That's a common occurance at blood banks.

[snippets69]

It ***** they dropped this on you like that.  The only obvious thing to do is wait for the confirmatory results to come back. You won't know until then.

[LIZZIE LOU]

HUH??????