Testing for established viruses: from the screening to the confirmation

Enzyme Immunoassays (EIAs) detecting antibodies (Ab) or antigen (Ag) used to avoid viral transmissions by transfusion, are the most suitable methods for blood screening because they are economical and adaptable to high-throughput testing. The continuing need to improve screening for HIV and HCV, led to develop Ag/Ab combination assays. The benefit of such assays has been widely demonstrated, since they are able to detect infected individuals in viremic pre-seroconversion phase, making them particularly attractive for ensuring blood safety when nucleic acid testing (NAT) cannot be implemented. In resource-limited countries, especially in remote-areas, blood safety is often based upon rapid tests (RT) which are easy to handle and can be used with whole blood. Nevertheless, the poor sensitivity of RT compared to EIAs could significantly impair blood safety. Because NAT could efficiently detect serologically negative donors, many developed countries have implemented this method in blood screening. NAT yields vary depending on epidemiological situations. NAT is subjected to false-negative results due to viral diversity or to extremely low viral loads, but also to false-positive due to sample cross-contamination. A repeatedly NAT reactive sample with multiplexed NAT is submitted to a virus-specific amplification testing to discriminate between the viruses, creating some problematic situations consisting of samples reactive with the screening assay but non-reactive with the discriminatory assay. Although the primary purpose of viral screening testing is to prevent pathogen transmission to recipients, confirmatory testing intends to manage donors with adverse test results and to guarantee an optimal blood supply. As EIAs show a low positive predictive value in low-prevalence countries due to frequent non-specific reactions, the use of confirmatory assays is justified. Nevertheless confirmatory tests are known to be less sensitive than EIAs in the early phase of infection and show also nonspecific reactivity leading to indeterminate results which create difficulties in managing donors. As in limited-resource countries, specific confirmatory assay cannot be used, alternate strategies for confirmatory testing have been proposed, as to simultaneously test blood with two assays and to exclude donors reactive with both, or to use screening test signal values to support confirmation. Although the direct detection of the virus through its genome might be considered as the best solution to ensure blood safety, the choice of blood screening strategy should be adapted to local epidemiology and organization constraints.