The diagnostic utility of Immunoglobulin G (IgG) avidity in distinguishing between past and acute infection of West Nile Virus (WNV).

Infection with West Nile Virus (WNV) can cause severe disease; however, due to immunoglobulin M (IgM) persistence and short viremia, confirmatory diagnosis of acute WNV infection often requires two consecutive samples and a laborious neutralization assay. Immunoglobulin G (IgG) avidity assay is being used to differentiate between acute and past infection of several viral diseases but not of WNV. To test whether IgG avidity can be utilized for WNV diagnosis, serum samples from 103 past and 84 acute WNV-infected persons diagnosed in our laboratory according to WHO guidelines were subjected to IgG avidity. ROC curve analysis was applied to measure the diagnostic accuracy of WNV IgG avidity and establish the optimal cut-off. Mean avidity and geometric mean neutralization titers (GMTs) from acute and past infected cases were 26% (95% CI: 21-30) and 183 (95% CI: 144-235) and 63% (95% CI: 59-67) and 110 (95% CI: 82-147), respectively. Optimal avidity cut-off level to distinguish between acute and past infection was 35% with sensitivity, specificity, PPV, and NPV of 81% (95% CI: 72-88), 88% (95% CI: 81-93), 55% (95% CI: 42-67), and 96% (95% CI: 94-98), respectively. Our results suggest that IgG avidity can rule out acute WNV infections in patients with positive WNV IgM and IgG antibodies and replace the neutralization assay as a confirmatory assay in most cases. By using an avidity assay as part of the routine diagnostic protocol of WNV, the diagnosis of WNV will be simplified, rapid, and would substantially reduce the workload and diagnostic difficulty of this flaviviral infection.IMPORTANCEDiagnosing West Nile Virus (WNV) infection typically requires two serum samples collected weeks apart and a labor-intensive confirmatory test using live virus. Here, we evaluated immunoglobulin G (IgG) avidity assay, which measures the binding strength between WNV antigen and IgG antibodies, as a simpler diagnostic approach using only one sample. Our findings showed that IgG avidity reliably identifies past WNV infections but is less effective in confirming the acute phase. Crucially, this method can rule out acute infections and allow for a rapid and definitive diagnosis in most cases. IgG avidity assays offer a streamlined alternative, reducing the need for complex tests and prolonged timelines.