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Risk of Transfusion

Today, the blood supply is extremely safe due to extensive donor screening and laboratory testing. However, a small number of transfused patients experience adverse transfusion reactions. An adverse reaction is defined as any unfavorable event that occurs during or after a transfusion. The cellular or fluid portions of the blood, anticoagulant-preservative solution, metabolic by-products, and circulating or contaminant microorganisms may cause adverse reactions. The estimated risks and etiologies of the most common transfusion reactions are listed below. Risk is expressed per unit of transfused blood rather than per patient. In this way, a patient’s actual risk can more accurately be estimated by multiplying the per unit risk times the number of units transfused.

Estimated Immunologic Risks



#Cases per Unit Transfused


Ab to donor proteins



Ab to donor IgA



Ab to donor WBCs



Ab to recipient WBCs


Wrong patient transfused

Human error



Ab to donor RBCs


Hemolytic, fatal

ABO incompatibility



Ab to donor cells


Graft vs Host Disease

HLA incompatibility



Today, TRALI and patient identification errors cause more transfusion related fatalities than HCV or HIV-1.Hepatitis C and HIV-1 viruses have been almost completely eliminated from the blood supply. These improvements have occurred largely through more comprehensive donor history screening, technological advances in infectious disease testing, and development of virus inactivation techniques for coagulation factor concentrates. Since the risks of transfusion transmitted infection are currently very low, it is difficult to accurately quantitate them. Consequently, risk estimates have been obtained with mathematical modeling techniques applied to data sets obtained from infectious disease testing of blood donors or follow-up investigations of selected transfusion recipients. The current estimated risks of transfusion transmitted infection, expressed per unit of blood received, are given in the following table.

Estimated Transfusion Risk


Per Unit Risk Estimate


1 in 2,135,000




1 in 2,990,000


1 in 352,000


1 in 1,935,000

West Nile Virus


Bacterial contamination of apheresis platelets

1 in 75,000


For the most important transfusion transmissible agents, Human Immunodeficiency Virus (HIV) and Hepatitis C virus (HCV), per unit risk is the same for each type of blood component transfused (i.e. red cells, platelets, FFP, cryoprecipitate). In contrast, for Human T cell Lymphotropic Virus Types I and II (HTLV-I/II) there is no risk of transmission from non-cellular blood products such as FFP or cryoprecipitate, since HTLV is highly leukocyte-associated.

It is unlikely that transfusion transmitted diseases will ever be completely eliminated for the following reasons:

  • Some high risk individuals continue to donate
  • Infected donors may be asymptomatic
  • Tests are not 100% sensitive
  • Human errors cannot be completely eliminated.

A nucleic acid or serological test may not become positive during the early stages of infection, known as the window period. For example, an individual exposed to HIV will not have a detectable viral load for at least 11 days following infection and antibody will not be detectable for 22 days. If an individual were to donate blood during this window period, their transfused blood components would be capable of transmitting HIV infection.

Platelets are stored up to five days at room temperature under aerobic conditions, which are ideal conditions for bacterial growth. Bacterial contamination has become one of the major risk factors to patient safety.

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