This study evaluated the performance of a new Food and Drug Administration-approved lateral-flow diagnostic screening test for qualitative detection of West Nile Virus (WNV) immunoglobulin M (IgM) in serum or plasma. Five public health laboratories across the United States performed retrospective testing on blinded serum samples from patients with physician reported diagnoses of WNV infection. The results of the RapidWN WNV IgM assay were compared with two commercially available WNV IgM enzyme-linked immunosorbent assays (EIA) and two public health-developed WNV-IgM tests. After discrepancies were resolved, the RapidWN WNV IgM EIA demonstrated a 98.8% sensitivity (range: 96.0-100%), a 95.3% specificity (range: 90.9-100%), a positive predictive value of 96.3% (range: 94.7-100%), and a negative predictive value of 98.4% (range: 95.5-100%), as compared to the predicate assays. The study results suggest that the RapidWN WNV IgM EIA is an effective, qualitative screening test that produces results comparable to that of predicate assays and can be employed rapidly to detect WNV IgM in patients suspected of having WNV infection.