There is a thorough screening process that will determine if your blood is suitable for donation. Physical exam also required during the screening process. They will not draw blood and just throw it away.
No. The process draws blood, spins the plasma separate from it, and returns your red blood cells to you. You will be screened in advance for certain infections (bad blood?) Of you know there's something wrong with your blood, don't waste your time or theirs.
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