Assays for detecting West Nile Virus antibodies in human serum, plasma, and cerebrospinal fluid

The rapid spread of West Nile Virus (WNV) across the North American continent has led to a need to understand what assays for WNV antibodies are available and how they are used as diagnostic and epidemiologic tools. In this article, we review six methods for measuring WNV antibodies in human serum, plasma, and cerebrospinal fluid. The complement fixation and hemagglutination inhibition assays were historically important; however, due to their low sensitivity, low specificity, and complex technical and reagent production issues, they are no longer in common use. The plaque reduction neutralization test is the gold standard for WNV antibody detection; due to its complexity and long turnaround time, however, it is increasingly reserved for establishing the presence of WNV infection in a geographic area and characterizing problematic samples. The immunofluorescence assay measures both IgG and IgM antibodies to WNV. Although historically considered insensitive, recent studies using commercially available slides have shown acceptable performance; the immunofluorescence assay is thus a cost-effective way to measure WNV antibodies in laboratories that routinely test small numbers of samples. The enzyme-linked immunosorbent assay (ELISA) format is the most popular method currently used to detect WNV IgG and IgM. Both indirect and monoclonal antibody-mediated antigen capture formats of IgG ELISAs have been described, whereas nearly all IgM ELISAs utilize the IgM capture format. Before 2000, WNV antibody ELISAs employed native WNV antigens; since then, there has been a dramatic shift toward using recombinant WNV antigens, particularly subviral particles containing the envelope protein. Like in the other assays mentioned, however, antibodies induced by other flavivirus infections may crossreact with both native and recombinant WNV antigens, necessitating concurrent measurement of antibodies to flaviviruses endemic in a given geographic area. The new microsphere immunoassay shows great promise as a sensitive, specific, and cost-effective method for simultaneously measuring antibodies to multiple flaviviruses. This method has also been used to characterize antibodies to nonstructural WNV proteins; these antibodies appear to be highly specific for WNV, and their measurement may soon be the test of choice for diagnosing WNV infection.