Performance characteristics of an in-house assay system used to detect West Nile virus (WNV)-specific immunoglobulin M during the 2001 WNV season in the United States

During the 2001 U. S. West Nile virus (WNV) season, 163 specimens were reactive in an in-house WNV-specific immunoglobulin M (IgM) screening enzyme-linked immunosorbent assay (ELISA) and were referred to either the Centers for Disease Control and Prevention or the appropriate state public health laboratory (CDC/SPHL) for additional testing. CDC/SPHL supplied results for 124 specimens that could be further evaluated in-house: 70 specimens were nonreactive in the CDC/SPHL WNV-specific IgM screening assay, and 54 specimens were reactive. These specimens were used to evaluate a modified in-house WNV-specific IgM ELISA that incorporated background subtraction to identify nonspecific reactivity and thus improve assay specificity. Of the 70 CDC/SPHL nonreactive samples, 49 (70%) were nonreactive in the modified ELISA; of the 54 CDC/SPHL reactive samples, 51 (94%) were reactive in the modified ELISA. Confirmatory studies performed by CDC/SPHL indicated that 38 CDC/SPHL screen-reactive specimens represented true WNV infection; all 38 specimens were reactive in the modified in-house WNW-specific IgM ELISA. These findings demonstrate that an in-house ELISA system for WNV-specific IgM effectively identifies patients with WNW infection.