Risk of Transfusion

50 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

Reaction	Etiology	#Cases per Unit Transfused
Urticaria	Ab to donor proteins	1:4000
Anaphylaxis	Ab to donor IgA	1:30,000
Febrile	Ab to donor WBCs	1:1000
TRALI	Ab to recipient WBCs	1:5,000
Wrong patient transfused	Human error	1:38,000
Hemolysis	Ab to donor RBCs	1:100,000
Hemolytic, fatal	ABO incompatibility	1:1,800,000
Alloimmunization	Ab to donor cells	1:200
Graft vs Host Disease	HLA incompatibility	1:1,000,000

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

Infection	Per Unit Risk Estimate
HIV-1	1 in 2,135,000
HIV-2	None
HTLV-I/II	1 in 2,990,000
HBV	1 in 352,000
HCV	1 in 1,935,000
West Nile Virus	None
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..

Last Updated on Wednesday, 13 July 2011