Collaborative study to evaluate a working reagent for West Nile virus RNA detection by nucleic acid testing.

BACKGROUND:  A nucleic acid test (NAT) assay reference reagent for West Nile virus (WNV) RNA, consisting of heat-inactivated WNV grown in tissue culture and diluted in pooled, negative human plasma, was evaluated and quantitated in a collaborative study in which 14 laboratories participated. STUDY DESIGN AND METHODS:  Participants were requested to assay serial half-log and log dilutions of the reagent to determine the RNA endpoint. A single endpoint for each such dilution series was calculated with the maximum likelihood method, which assumes that the probability of a positive result at a given dilution follows a Poisson distribution. The calculated endpoint was used to give an estimated “NAT-detectable units per mL” (not necessarily equivalent to genome equivalents/mL or copies/mL). The assays used by participants included qualitative and quantitative NAT assays and both the commercial WNV assays (Chiron and Roche). RESULTS:  The estimated number of detectable units per mL for the 14 laboratories varied from log 2.0 to 3.0 with the exception of two outliers. The overall mean titer for all the assays was log 2.52 detectable units per mL (330 detectable units/mL). Multiple testing of individual vials by two laboratories indicated that there was no evidence of vial-to-vial variation in WNV content of the reference reagent. CONCLUSION:  A reference reagent for WNV NAT assays has been established. The mean titer of the reagent, with the results from 14 laboratories, was 330 detectable units per mL.