Performance evaluation of a new routine HCV core Ag assay for viraemia detection in anti-HCV positive samples.

Background Nucleic Acid testing (NAT) is currently used to detect viremia in individuals identified as anti-HCV positive. Although exquisitely sensitive, NAT is prone to contamination, requires special infrastructureltraining, and a newly drawn sample that may delay the generation of a final report. A new EIA test capable of detecting and quantifying the core Ag of HCV in the presence of anti-HCV has recently become commercially available. The increased stability of the core Ag over RNA, allows for this assay to be performed on the same sample obtained for anti-HCV testing. Aims: Evaluate the performance of a new core Ag assay in detecting HCV viremia in stored anti-HCV positive samples. Methods: Serum/plasmasamples from the following populations were included 1) 308 anti-HCV negative samples from blood donors which were prospectively collected and stored at -20°C until tested, 2) 310 anti-HCV negative samples from hospitalized patients, or from patients attending our hospital, which were prospectively collected and stored at -2OOC until tested, 3) 405 anti-HCV positivelHCV RNA negative, and 604 anti-HCV positivelHCV RNA positive samples also prospectively collected and stored at -80°C until tested. Supplemental HCV antibody testing was performed using the RIBA 3.0 strip immunoassay (SIA) (Chiron, Emeryville, CA). HCV RNA was investigated using the COBAS Amplicor HCV v2.0 assay (Roche Diagnostics, Branchburg, NJ). The presence of HCV core Ag was determined using the trak-C assay (Ortho Clinical Diagnostics, Raritan, NJ). Four reference serum samples (AS1-AS4) ranging in core Ag concentrations were analyzed to determine trak-C’s intra- and inter-assay precision. Results: The core Ag intra- and inter-assay precision values obtained with the four reference sera were the following: AS1: 5,4%, 11,9%; AS2: 7,9%, 21,4%; AS3: 9,9%, 21,5%; and AS4: 18,9%, 17,0%. The limit of detection of the trak-C assay was 1.5 pg/mL as established by the manufacturer. Taking into account the assay’s lower end inter-assay precision (17,0%), we established the following gray zone: 1.5 pg/mL 2 0,26 pg/mL. A sample was considered reactive for core Ag when its concentration was y~1,8p g/mL, negative when its concentration was < 1,2 pg/mL and dubious when its concentration was between 1,2 pg/mL and 1,8 pg/mL. Specificity values based on initially reactive (IR) results from blood donors and hospitalized anti HCV negative patients were 99,7%, and 100% respectively, while the specificity based on repeatedly reactive (RR) results was 100% in the first population. Two out of 405, and 0 out of 405, anti-HCV positivelHCV RNA negative samples resulted IR and RR for core Ag respectively yielding specificities of 99,5% and 100%. Finally, 572 out of 604 (94,7%) anti-HCV positivelHCV RNA positive samples resulted core Ag reactive, while 4 out of 604 (0.66%) fell within the gray zone. A similar distribution of RIBA 3.0 reactivity patterns was observed between HCV RNA and core Ag positive samples. Conclusions: The combination of sample requirement, easiness of use, and clinical performance (specificity loo%, and sensitivity 947%) places the trak-C assay as a reliable alternative for viremia detection in most anti-HCV positive samples, reducing significantly the use of NAT.