Assessing the impact of HBV NAT on window period reduction and residual risk

Abstract

The incidence window period model for estimating residual risk was developed in the mid-1990s and in the last 5 years has been widely used internationally to estimate the risk of transfusion-transmission of agents for which donor blood screening is in place (e.g. HIV, HCV, and HBV) (Schreiber et al., 1996; Kleinman et al., 1997; Glynn et al., 2002; Coste et al., 2005). The premise of the model is simple: the probability that a potentially infectious donation will be released into the blood supply (termed residual risk, to indicate that it is risk persisting despite laboratory screening) is estimated by multiplying the incidence of new infection (seroconversion rate in a blood donor population over a specific time interval) by the average time interval that a seroconverting donor is capable of transmitting the infection (i.e., the length of the infectious pre-seroconversion window period). The model can also be used to project the yield of a new assay by multiplying incidence by the differential window period between the new and previously used donor screening assays. A basic assumption of the model is that a donor is equally likely to donate on any given day during the pre-seroconversion and post-seroconversion window periods, i.e. that donation behavior is not affected by the acquisition of acute infection. Incidence data are obtained from direct observation of blood donor populations and are measured as the number of newly infected (seroconverting) donors detected divided by the observed person-years at risk; therefore this direct incidence measure applies only to repeat donors who made more than one donation during the study period. With the advent of minipool (MP) NAT testing for HIV and HCV,