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


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									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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