West Nile Virus (WNV) infection is a notifiable disease. Cases of WNV disease are reported by state health departments to the Center for Disease Control and Prevention through ArboNET, an internet-based, passive surveillance systems. Cases reported to ArboNET must have clinical evidence of compatible illness and laboratory evidence of recent WNV infection. Based on patients' clinical signs and symptoms, WNV cases are classified as neuroinvasive disease or nonneuroinvasive disease. A report on a nationwide investigation into an increase in false-positive test results obtained with a commercially available WNV immunoglobin M capture enzyme-linked immunosorbent assay is presented.
458 MMWR May 8, 2009 8. Young LC, Dwyer DE, Harris M, Guse Z, Noel V, Levy MH; Prison Influenza United States. These assays are labeled for use on serum to aid Outbreak Investigation Team. Summer outbreak of respiratory disease in an Australian prison due to an influenza A/Fujian/411/2002(H3N2)-like in a presumptive diagnosis of WNV infection in patients who virus. Epidemiol Infect 2005;133:107–12. have clinical symptoms consistent with neuroinvasive disease. According to product inserts (3–6), all positive results obtained with these assays should be confirmed by plaque reduction neutralization test (PRNT) or by using current CDC guidelines False-Positive Results with a for laboratory diagnosis of this disease (7). Commercially Available West Nile Initial Investigation Virus Immunoglobulin M Assay — In summer 2008, three state health departments indepen- United States, 2008 dently contacted CDC regarding positive WNV IgM antibody test results in patients who lacked clinical or epidemiologic In September 2008, CDC, the Food and Drug Administration evidence of WNV infection. All of these tests results origi- (FDA), and state health departments began a nationwide inves- nated from one large commercial laboratory that was using the tigation into an increase in false-positive test results obtained PanBio WNV IgM ELISA test kit manufactured by Inverness with a commercially available West Nile virus (WNV) immu- Medical (Princeton, New Jersey). On September 5, 2008, the noglobulin M (IgM) capture enzyme-linked immunosorbent New York State Department of Health’s Wadsworth Center assay (ELISA). The investigation revealed that, in the United laboratory reported that 13 (86%) of 15 specimens testing States, one lot of the commercially available test kits was the positive for WNV IgM antibodies at the commercial laboratory source of the false-positive results (1). That lot was recalled, in August were negative upon retesting at the state laboratory. and a second lot distributed outside the United States also was On September 10, CDC notified all state health departments recalled (1). During July 1–September 30, 2008, the kit lot of the potential problem and initiated an investigation into implicated in the United States resulted in positive tests on the cause of the false-positive test results (1). 568 specimens collected from 518 patients in 42 states and In late September, one of the affected commercial labora- the District of Columbia (DC). A total of 166 (29%) speci- tories sent a convenience sample of 64 specimens that had mens were retested at CDC, and 119 (72%) had false-positive yielded positive or negative WNV IgM antibodies results to results. A higher false-positive percentage were found among CDC and the kit’s manufacturer for retesting. This evaluation patients without evidence of neuroinvasive disease (77%) than identified two lots of the kit with higher false-positive rates patients with evidence of neuroinvasive disease (47%). Of the (20% and 56%) than the expected rate calculated from data in 518 patients, 249 (48%) had been reported to CDC as persons the package insert (2% [95% confidence interval = 0%–9%]). with WNV disease; however, only 45 (18%) had confirmatory On October 8, these two lots were recalled voluntarily by the testing that supported their inclusion in national surveillance manufacturer. The lot with the 56% false-positive rate had been data. Commercially available WNV test kits should be used distributed to four laboratories in the United States and was to determine a presumptive diagnosis o
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