Non-A, non-B hepatitis has for years been recognized as a major complication of blood transfusions. Indeed, the risk for transfusion of hepatitis in patients receiving multiple transfusions was at least 7-10% before 1980 (1). With the recent isolation of a previously unknown virus, named hepatitis C virus (HCV), the great majority (75-85%) of non-A, non-B hepatitis cases are now being attributed to this agent.
HCV is a single-stranded RNA virus at least 10,000 nucleotides long and smaller than 80 nm that is contained in an envelope and is related to the flaviviruses. It was first identified in 1989 through molecular cloning using immunoscreening of expression libraries prepared from the cDNA of a nucleic acid extract obtained from the highly infectious plasma of a chimpanzee (2). The HCV genome codes for both structural and nonstructural proteins that can serve as determinants for the immune response of the infected host (3).
Transmission of HCV
The transmission of HCV occurs efficiently
through blood or blood products such as red blood cells, platelet concentrates,
fresh frozen plasma, and cryoprecipitate. Infection through Rh(D)
immunoglobulin or clotting factor concentrate has also been reported (4,5).
Less efficient routes of infection include transmission through sexual contact
and through perinatal infection (transmission through perinatal infection has
not been described to date but may occur in rare instances). In general, an
increased risk for HCV infection has been noted in transfusion and transplant
recipients, dialysis patients, intravenous drug abusers, and persons with
unexplained elevation of serum alanine aminotransferase (ALT) levels. A
potential risk exists also for health care workers, particularly after
needle-stick injury or other blood contact.
HCV Screening Methods
The prevalence of HCV antibody among blood
donors is between 0.5% and 1.5% in the U.S. and different European countries;
this is based on results obtained using a recently introduced anti-HCV
screening test (Ortho Diagnostics, Raritan, NJ) developed by Kuo et al. (6). Of
66,110 M. D. Anderson Cancer Center blood donors (May 1, 1990-March 14, 1992)
screened with the test, 432 (0.65%) were anti-HCV positive. However, with a
recently improved screening test (Ortho Diagnostics), only 25 (0.5%) of 4,575
blood donors (March 15, 1992-April 14, 1992) tested anti-HCV positive,
indicating a slight reduction of 0.15% in the number of donors testing anti-HCV
positive.
Currently available methods for detecting anti-HCV are enzyme immunoassay (EIA) screening tests licensed by the Food and Drug Administration (FDA) and supplemental recombinant immunoblot (RIBA), neutralization, and polymerase chain reaction (PCR) assays unlicensed by the FDA. EIA tests used for normal blood-donor screening are commonly based on recombinant antigens placed in microtiter plates that react with HCV antibody present in serum or plasma. These antibodies are detected with an enzyme-labeled anti-human immunoglobulin. The most commonly used supplemental test (Ortho Diagnostics) is a four-antigen RIBA incorporating the following antigens: c100-3, 5-1-1, c33c, and c22-3.
Unfortunately, EIAs show a significant number of false-positive anti-HCV results (30-60%) when specimens are subjected to supplemental testing. In contrast, false-positive results are very few or nonexistent when specimens from patients with anti-HCV-positive non-A, non-B hepatitis and from anti-HCV-positive donors with elevated ALT levels are subjected to PCR assays. In such cases, there is a high concordance (up to 100%) between the presence of anti-HCV and HCV RNA, whereas in random blood donors positive for anti-HCV by EIA and supplemental tests, the concordance between anti-HCV and PCR-detectable HCV RNA is much lower (only 50-70%). It is important to note, however, that appropriate specimen processing and storage methods are essential for the subsequent detection of HCV RNA in serum or plasma specimens (7).
Delayed seroconversion in hepatitis C-infected donors further limits the detection of HCV-infected donor blood by current screening tests. Delayed seroconversion may affect detection since anti-HCV can be detected in only 60% of cases within 15 weeks and in 90% of cases within 6 months after elevation of ALT (8). In contrast, HCV RNA can already be detected in plasma or serum after 2 weeks. Furthermore, hepatitis C infections are frequently anicteric; only 50% of patients with acute hepatitis C might form anti-HCV, but, if so, they can rapidly lose the antibody. Therefore, the commonly used donor screening tests might not prevent the transfusion of blood from potentially infectious persons at a later date. However, patients with chronic non-A, non-B HCV hepatitis remain anti-HCV positive and are usually detected by regular donor testing.
Need for a More Sensitive and Accurate HCV Screening Assay
The
introduction of HCV antibody screening of all donor blood represented a major
step in the prevention of transfusion-associated HCV hepatitis. Accordingly,
the risk for HCV seroconversion after transfusion significantly decreased from
3.84% to 1.54% and 0.57% per patient following the introduction of ALT and
anti-hepatitis B core antigen (HBc) testing in 1986 and anti-HCV screening in
May 1990, respectively. This corresponds to a decline in risk from 0.45% to
0.19% and 0.03%, respectively, per unit transfused (9).
Nevertheless, a more sensitive and accurate HCV-antibody and confirmatory test is urgently needed since HCV infection progresses to chronic hepatitis in more than 50% of infected patients and since a significant number of those patients subsequently develop liver cirrhosis and sometimes hepatocellular carcinoma (HCC). A causal relationship between HCV infection and HCC is presently being debated and is far from established, but still, the possible connection argues for such a test. Furthermore, rapid and sensitive assays developed to detect either HCV itself or a viral antigen could be used for blood donor screening. Both types of tests would first and foremost help to prevent potentially infectious blood from being transfused and would allow more informative counseling of HCV-positive donors. The same tests would be equally important for diagnosing HCV hepatitis and for studying the possible role of HCV in the development of HCC (10,11).
References
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