West of Scotland Specialist Virology Centre, Gartnavel General Hospital, 1053 Great Western Road, Glasgow, United Kingdom. Electronic address: Samantha.Shepherd@ggc.scot.nhs.uk.
DBS testing has been used successfully to detect HCV antibody positive individuals. Determining how long someone has been infected is important for surveillance initiatives. Antibody avidity is a method that can be used to calculate recency of infection.
A HCV avidity assay was evaluated for both plasma and DBS.
To measure antibody avidity a commercial HCV ELISA was modified using 7M urea. The plasma samples were split into: group 1 (recently infected N=19), group 2 (chronic carrier N=300) and group 3 (resolved infection N=82). Mock DBS made from group 1 (N=12), group 2 (N=50), group 3 (N=25) and two seroconverter panels were evaluated. 133 DBS taken from patients known to have a resolved infection or be a chronic carrier were also tested.
The avidity assay cut-off was set at AI≤30 for a recent infection. Using sequential samples the assay could detect a recent infection in the first 4-5 months from the point of infection. Most of the false positive results (AI<30 among cases known not to have had recent infection) were detected among known resolved infections, in both the plasma and DBS; as a result, a testing algorithm has been designed incorporating both PCR and two dilution factors. The sensitivity and specificity of the assay on plasma was 100% and 99.3%, respectively, while DBS had 100% sensitivity and 98.3% specificity.
The HCV avidity assay can be used to distinguish between chronic and recent infection using either plasma or DBS as the sample type.