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A commercially available West Nile Virus test produced a substantial number of false-positive test results and affected the number of West Nile Virus (WNV) cases reported in the United States in 2008, says a study published Thursday.
One lot of the WNV Immunoglobulin M (IgM) ELISA kit used at four laboratories resulted in positive tests from 518 patients in 42 states. Retesting of available samples showed a 72 percent false-positive rate. The majority of those false-positive results were from people without symptoms of neuroinvasive disease.
The lot of the test kit, labeled for use on serum to help aid in suspected cases of WNV neuroinvasive disease, was used starting in July 2008 and was recalled after a multistate investigation into an increased number of false-positive WNV test results.
The indicated use of commercially available test kits should be considered when requesting testing and interpreting results. Any positive results should be confirmed by a state health department or the U.S. Centers for Disease Control and Prevention, said the authors of the report, which was published in the CDC's Morbidity and Mortality Weekly Report.