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Physicians prescribing transfusion are commonly faced with detailed questioning from anxious patients and family members about blood safety.  Although there are numerous facets to these questions, the major concerns are the risks of contracting AIDS or hepatitis through transfusion. 

As a result of careful donor selection and sensitive testing procedures, it can be unequivocally stated that the blood supply is safer than ever.  To support this, recent studies have determined the risk of infection per unit of blood transfused.  These data are summarized below. 

Transfusion-Assoc. HIV Infection (TA-HIV)

TA-HIV infection has been the subject of intense scrutiny.  With the advent of specific donor questioning about “high risk” activities and mandatory testing of all blood products, there has been a dramatic decrease in the incidence of TA-HIV infections.  The EIA screening test, which measures antibodies to HIV, is exquisitely sensitive.  A consequence of the extreme sensitivity of the test is a high incidence of false positive reactions.  At Central Blood Bank in 1992, only 7/300 EIA positive tests for HIV were actually confirmed (by Western blot analysis) to be infected.  Nevertheless, it is universally agreed that, for blood safety, it is better to err on the side of excluding some uninfected donors then risk the possibility of missing an infected unit.  No unit testing positive by EIA for HIV is used for transfusion. 

Initially, there was concern that some infected individuals might experience a long interval between infection and seroconversion (window period).  This was supported by sporadic reports of prolonged antibody negative, virus positive interval in high risk subjects.  However, none of these subjects were, in fact, infected.  The false positive tests for the presence of HIV resulted from faulty testing procedures.  In individuals for whom the date of infection can be determined, seroconversion invariably occurs within six months of exposure. 

There have been several studies defining the time sequence between infection and seroconversion.  With the currently employed HIV ˝ assay, seroconversion typically occurs within 33 to 40 days after infection.  There have been discussions as to whether this interval could be significantly reduced by an HIV antigen test.  Recent data indicate that such as test would marginally shorten the interval between infection and detection (by 4 to 5.5 days).  However, the utility of this test has been questioned.  Surveys indicate that it would detect less than one infected unit in two million blood donations. 

HIV-2, a virus closely related to HIV-1, also causes AIDS.  This virus is endemic in Western Africa; it is extremely rare in the United States.  At present, it does not pose a significant health risk.  There have been no cases of transmission by blood or blood products.  However, it is suspected that the prevalence of this viral infection will increase.  Despite its rarity, all blood donations are tested for the presence of antibodies to HIV-2. 

At the Ninth International AIDS Conference, it was reported that a small number of individuals had a syndrome consisting of decreased CD4 cell counts without evidence of HIV infection.  These findings raised the possibility of a new virus capable of causing immunosuppression. 

Epidemiological and clinical studies identified a small heterogeneous group of individuals with unexplained low CD4 counts in the absence of HIV infection and other known causes of immunosuppression.  Less than half had risk factors for AIDS and no route of transmission could be identified.  Thorough virological studies failed to detect a pathogen.  There is no evidence that this syndrome is transmitted by blood products.

Transfusion-Assoc. Hepatitis

The other major infection transmitted by transfusion is hepatitis.  Over the past 20 years, there has been a dramatic decrease in the incidence of post-transfusion hepatitis (PTH).  Prior to 1970, some blood centers reported that >20% of transfused recipients developed PTH.  The change from paid to volunteer donors, the addition of intense donor screening procedures have reduced the incidence of virally-induced PTH to <1%. 

Almost all PTH are caused by either hepatitis C (HCV) and hepatitis B (HBV) viruses.  The procedures for screening blood for HBV are sensitive and cases of PTH-HBV are now extremely rare.  All units are tested for hepatitis B surface antigen (HbsAg).  A positive test indicates an infectious carrier.  In addition, donors are tested for hepatitis B core antibody (HbcAb).  The presence of this antibody may indicate previous infection or the chronic carrier state.  It is also a useful surrogate marker for HCV infection.  It should be noted that among blood donors, there is a high incidence of false positive HbcAb tests.  This should be considered in donor counseling.  At present, there are no means for re-entering subjects deferred for positive HbcAb tests. 

The principal cause of PTH is HCV.  Although all donations are tested by a sensitive second generation test for HCV antibodies, there may be a long interval between infection and seroconversion.  The window period averages approximately four weeks.  However, a small percentage of infected subjects show delayed seroconversion; some may not develop detectable antibodies for periods up to one year after infection.  HCV has been found to cause chronic liver disease in approximately 50% of infected patients. 

Hepatic inflammation of HCV may be detected by elevations of alanine amino transferase (ALT) levels.  All blood products are tested for ALT and units with elevated values are discarded.

HTLV I/II

Another group of viruses that can be transmitted by blood products are the retroviruses, HTLV-I and HTLV-II.  These viruses are distinct from HIV and do not cause AIDS.  However, a small percentage of HTLV-I infected individuals may develop either an acute T cell leukemia (ATL) or a neuromuscular disease called tropical spastic paraparesis or HTLV-I-associated myelopathy (TSP/HAM).  The incidence of either complication is extremely low.  In Japan, where HTLV-I is endemic, about 3% of infected individuals develop ATL and <1% TSP; ATL is known to occur after prolonged latent (>20 years) periods. 

HTLV-II, a closely related virus, is commonly found in intravenous drug addicts.  To date, this virus has not been definitely associated with any disease.  The screening test does not distinguish between these two viruses.  In counseling patients infected with either HTLV-1 or HTLV-II, it should be stressed that these are sexually transmitted infections. 

There are several other infectious agents that are potentially transmissible through blood products.  Most are extremely rare (incidence of <1:1,000,000).  An exception is cytomegalovirus (CMV), which poses little risk to the recipient except in the neonatal period and in organ or bone marrow transplant recipients.  Another concern is the possibility of bacterial contamination of a blood product.  This is a problem primarily encountered with platelets stored at room temperature.

For questions regarding the “The Safety of Blood”, please contact Joseph E. Kiss, M.D.