Currently blood donors answer a questionnaire where they are asked about travel, medical history, and high risk behavior. Then they undergo a physical evaluation to obtain blood pressure, temperature, and a finger stick to measure hemoglobin. 

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Then after the blood is donated, samples are taken from the blood unit to test for blood typing, blood group antibody screening, infectious disease antibody testing, and infectious disease nucleic acid testing.

 

Although this greatly reduces risk of transfusion-transmitted disease, there is still risk, especially for novel pathogens where a screening test has not yet been created.