Evaluation of indeterminate c22-3 reactivity in volunteer blood donors

Background: Approximately 25 percent of blood donor sera that are repeatably reactive for hepatitis C virus (HCV) on second-generation enzyme immunoassay (EIA 2.0) are indeterminate on second-generation recombinant immunoblot assay (RIBA 2.0), and over 76 percent of these results are due to single reactivity to the HCV recombinant antigen c22- 3. Study Design and Methods: Data are presented on 46 volunteer allogeneic blood donors who were reactive on EIA2.0 and c22-3 indeterminate in RIBA 2.0. Index and follow-up samples were evaluated by using a panel of five synthetic peptide EIAs, a prototype strip immunoblot assay that uses synthetic peptides in addition to recombinant protein (RIBA 3.0), and polymerase chain reaction (PCR) for HCV RNA. Results: All 46 donations had normal alanine aminotransferase values; only 2 (4.3%) reacted for antibody to hepatitis B core antigen. With a panel of 12 synthetic peptides spanning the entire sequence of the c22-3 recombinant antigen, 33 plasmas (72%) reacted to one peptide or none, including 19 plasmas with reactivity restricted entirely to the N-terminal peptide (1–15 amino acids) of c22-3. With RIBA 3.0, 28 donations (61%) were nonreactive, including 25 that reacted with one peptide or none in EIA. Of these 25 plasmas, 18 reacted with the N- terminal sequence only. All 46 index donations were tested by PCR; the single PCR-positive unit reacted with four HCV peptides, was positive by RIBA 3.0, and reacted for antibody to hepatitis B core antigen. Twenty-six index donors were successfully recalled 3 to 7 months after their index donation. None seroconverted to positivity in RIBA 2.0, 1 was nonreactive, and 25 remained positive for c22-3 only. The restricted epitope reactivity in peptide EIA and RIBA 3.0 was maintained over time in all cases. All 26 of the follow-up samples tested negative by PCR. Conclusion: On the basis of the restricted peptide reactivity and PCR negativity of index and follow-up samples, it is concluded that the majority of c22-3 RIBA 2.0-indeterminate results are due to nonspecific cross-reactivity to restricted (principally, N-terminal) regions of HCV core antigen.