Informed consent, risk, and blood transfusion.

Informed consent for blood transfusion has become a necessity in light of the known risks associated with this service. All transfusion services should institute written informed consent that clearly defines the patient's options, including the use of homologous blood, autologous blood, and directed donations. The risk of transfusion with an infectious blood product is dependent on the number of donors per recipient and the prevalence of undetected, contaminated blood in the tested blood supply. The chance that an adverse transfusion will occur can be calculated by use of these variables. Comparative risks can be explained to patients, thereby providing an understanding of the transfusion risk of human immunodeficiency virus, the human T-cell leukemia virus, and the agent of non-A, non-B hepatitis (hepatitis C).