February, 1995


Katherine A. Anderegg, M.D., Transfusion Medicine Fellow

Joseph E. Kiss, M.D., Medical Director, Hemapheresis and Blood Services


The public perception of the risk of viral transmission from a blood transfusion remains substantially higher than the actual magnitude of the risk. Anxious patients and their families often question the safety of a blood transfusion. This occurs despite numerous studies documenting that the risk of infection is very small. Further, the risk is progressively decreasing with improved screening and testing methods. To aid clinicians in guiding patients to make informed choices concerning transfusion, recent data on the risk of viral transmission, nationally and locally, are presented.


Incidence of HIV Positive Units:
U.S. vs. Central Blood Bank Donors

The annual incidence of community and directed donor units with Western blot confirmed HIV positivity has declined from 12:100,000 donors in 1986 to the current level of 1-2:100,000. Thus, there has been nearly a 10-fold reduction (see graph). This decline attests to the value of more rigorous donor screening measures, directed primarily at ascertaining behavioral risk factors associated with HIV. The extremely low frequency of HIV in volunteer blood donors is even more striking considering the fact that 1 in 250 Americans is estimated to be infected with HIV.

Because of present screening procedures, the only risk of HIV transmission results from blood donations collected during the "window period", the interval between infection and seroconversion. Conventional donor testing will not detect antibodies to HIV in this phase of the illness. With current generation EIA-3 testing procedures, this "window period" averages 22-25 days. This is nearly a 50% reduction from the 45 day window period observed with previously used, less sensitive tests. In the future, this interval may be further reduced if molecular based screening methods are used to detect the virus (1). The actual risk of a recipient contracting HIV or other viral infections can be calculated as the product of the prevalence of the disease in the donor population and the chance that the donor is in the window period at the time of the donation:

Risk =

prevalence x window period (in days)

Factoring in the window period, the overall risk of HIV infection in the U.S. from transfusion is estimated at 1:340,000 units and at 1:280,000 in the Northeast. At least 20% of these HIV infectious units are excluded from use because of other positive screening tests. Thus, the risk of contracting HIV from a blood transfusion is less than 1:420,000 units for the U.S. and >1:346,000 for the Northeastern part of the country (2,3). In the Central Blood Bank donor population, there were only three HIV confirmed positive donors among the 163,849 units collected from July 1, 1993 to June 30, 1994. This translates to a risk of 1:800,000 units. If 20% of the antibody negative HIV positive units would be excluded because of other screening tests, the true risk for transfusion recipients in the tri-state region is 1:960,000 units (see Table). These figures are over 50% less than the published national and Northeast region values, making Pittsburgh among the safest areas in the country with respect to transfusion-transmitted HIV.

At one time, there was a fear that certain HIV infected patients may have a very prolonged window phase. However, accumulated data indicate that a long antibody-negative interval does not occur (2). Virtually all infected individuals will seroconvert within 3 months, and all infected individuals will have a positive test within 6 months.


Other Viruses: Risk of Transfusion Transmission from U.S. Donors Compared to Pittsburgh Donors

Although HIV causes the most concern for patients and families, it is among the least commonly encountered viruses transmitted by blood products. Listed in the Table is a comparison of the infectious risk for transmission of HIV, Hepatitis B (HBV), Hepatitis C (HCV), and HTLV-1 from red cell units in the Pittsburgh area with current national figures. These calculations estimate 100% infectivity. The window period for HCV varies widely (15-180 days); as such, a modal number of 90 days is used. Because the second generation HCV screening test has a high incidence of false positives, it appears that less than 50% of positives are truly infected (4,5). HBV is screened by an antigen test which actually detects the virus. It is unclear as to when the test becomes positive after infection so no window phase has been used for calculations. Insufficient data are available to accurately approximate the window for HTLV-1. At least 50% of the positive HTLV-1 tests are due to infection with HTLV-2; the number of positives in the HTLV-1 category has been reduced to correct for this.


Risk per Unit Transfused

Virus US (3,6,7,8) Pittsburgh
Region (1994)
HIV 1/420,000 1/960,000
HBV 1/800,000 1/800,000
HCV 1/2,000-1/6,000 1/4,300
HTLV-1 1/70,000 1/82,000

To put these risks in perspective, the chances of developing fatal anaphylaxis from a single dose of penicillin (1:50,000) is nearly twenty times that of contracting HIV from a CBB blood transfusion. Yet, few patients would question the risks of a ten day course of this antibiotic. Explaining transfusion risks in terms of such familiar activities may help to alleviate the excessive anxiety that some individuals experience at the news that they or a relative needs a transfusion. While good clinical judgment in weighing the risks and benefits of a blood transfusion is necessary, it is important to recognize that it is among the safest invasive medical procedures performed today.



For questions or further information please contact
Joseph E. Kiss, M.D. at (412) 209-7326

e-mail: jkiss@itxm.org


Copies of the Transfusion Medicine Update can be obtained by contacting
Deb Small - (412) 209-7320
email: dsmall@itxm.org