WHO Collaborative Study to Establish a Replacement WHO International Standard for HCV RNA NAT Assays

Summary In the collaborative study to establish the first International Standard for HCV RNA, there was no significant difference in potency between the two materials, AA and BB, which had been prepared from the same stock material but had been lyophilised in different runs. From the outset it was anticipated that sample BB would be considered as the candidate second International Standard and that a full collaborative study would not be considered essential as the candidate material had been fully characterised in the initial study to establish the first HCV International Standard. Three laboratories using different NAT assays participated in a small collaborative study to assess the potency and the stability of the candidate replacement standard, sample BB in the original study, NIBSC code 96/798, compared with the current HCV International Standard (sample AA in the original study; NIBSC code 96/790). The current study investigated the relative stability of the two materials. There were no significant differences between the two materials stored at -20oC and no evidence of degradation over 5 years. Both materials also had similar profiles of degradation at higher temperatures. Based on this study, candidate material 96/798 is proposed as the second International Standard for HCV RNA NAT assays and assigned the same unitage as the first International Standard. The proposed second International Standard for HCV would therefore have a potency 50,000 IU/vial, which is equivalent to 105 IU/ml when a vial is reconstituted in 0.5ml.