Background: The hepatitis C virus (HCV) is now known to be the chief cause of transfusion-associated non-A, non-B hepatitis, but the prevalence of HCV among blood donors and the frequency of transmission by blood transfusion are unknown.

Methods: To assess the sensitivity and specificity of a test for antibody to HCV, we tested serum samples from participants in a large study of transfusion-associated hepatitis. Samples were obtained prospectively from consecutive adults undergoing open-heart surgery in Spain, but were tested retrospectively, after the antibody enzyme immunoassay for anti-HCV became available.

Results: Of 280 transfusion recipients given a total of 1109 units of blood, 27 (9.6 percent) had transfusion-associated non-A, non-B hepatitis (mean follow-up, 52 weeks) and 24 of the 27 seroconverted to anti-HCV-positive, whereas only 2 (0.8 percent) of the remaining transfusion recipients seroconverted. Among the 1044 donor specimens available for testing, 16 (1.5 percent) had anti-HCV antibody. Only 1 additional seropositive donor was found when 44 implicated donors who had been seronegative were retested 9 to 12 months later. Of the 16 recipients of anti-HCV-positive blood, 14 (88 percent) had transfusion-associated hepatitis and seroconverted to anti-HCV-positive. The remaining two recipients had neither hepatitis nor anti-HCV antibody. Among 25 patients with non-A, non-B hepatitis for whom all transfused blood was tested, 14 had received blood positive for anti-HCV.

Conclusions: About 90 percent of blood donors with antibody to HCV have infectious virus in their blood. The screening of blood donors for anti-HCV antibody should prevent about half the cases of transfusion-associated hepatitis, but the donors with infectious virus who are anti-HCV-negative may remain seronegative for prolonged periods.