An algorithm for confirming screen reactivity in blood donors in enzyme immunoassays for antibodies to hepatitis C virus.

Three commercial second generation enzyme immunoassays (EIA) and a second generation immunoblot assay (RIBA-II) for detecting antibodies to hepatitis C virus (HCV) were evaluated in a study of confirmatory testing using sera referred by six Regional Blood Transfusion Centres (RTC). Of a total of 490 samples, 203 were negative in the same EIA as that used by the RTC and of these 162 were negative in all three EIAs. The RIBA-II immunoblot test was performed on all samples and 359 were negative, 69 were indeterminate and 62 reactive. We found a close relationship between RIBA-II reactivity, the reactivity of a sample in all three EIAs and the mean of the test/cut-off ratios of all three EIAs performed on that sample. When this was evaluated by PCR for HCV RNA, we found an association between the mean test/cut-off ratio and the probability that an immunoblot reactive or indeterminate sample was PCR positive. A mean ratio of greater than 5.0 in a RIBA-II reactive sample was associated with a 100% probability (16/16 tested) of being PCR positive. These observations should be extended by testing RIBA-II negative samples by PCR so that simplified algorithms for anti-HCV testing can be developed.