The little bit of blood that's taken up in the small vials are quite minimal to
your total blood supply throughout your body. Consider those people who
give whole pints of blood at blood banks - sometimes several pints at one
time. They still have plenty of blood left after their donation.
Sometimes blood tests need to be done more than once or maybe more than
one kind of test must be run. Having an extra supply of a person's blood is
a good idea as the person won't have to return for another finger prick, etc.
Nothing to worry about!