False-Positive Results with a Commercially Available West Nile Virus Immunoglobulin M Assay — United States, 2008

In September 2008, CDC, the Food and Drug Administration (FDA), and state health departments began a nationwide investigation into an increase in false-positive test results obtained with a commercially available West Nile virus (WNV) immunoglobulin M (IgM) capture enzyme-linked immunosorbent assay (ELISA). The investigation revealed that, in the United States, one lot of the commercially available test kits was the source of the false-positive results. That lot was recalled, and a second lot distributed outside the United States also was recalled. During July 1--September 30, 2008, the kit lot implicated in the United States resulted in positive tests on 568 specimens collected from 518 patients in 42 states and the District of Columbia (DC). A total of 166 (29%) specimens were retested at CDC, and 119 (72%) had false-positive results. A higher false-positive percentage were found among patients without evidence of neuroinvasive disease (77%) than patients with evidence of neuroinvasive disease (47%). Of the 518 patients, 249 (48%) had been reported to CDC as persons with WNV disease; however, only 45 (18%) had confirmatory testing that supported their inclusion in national surveillance data. Commercially available WNV test kits should be used to determine a presumptive diagnosis of WNV neuroinvasive disease. These kits should not be used to test specimens from persons without compatible illness, and any positive result should be confirmed by additional testing at a state health department or CDC.