United States of America - The Food and Drug Administration (FDA) has approved the first test for use in the clinical laboratory diagnosis of West Nile virus infection. West Nile Virus IgM Capture ELISA® is intended for use in patients with clinical symptoms consistent with viral encephalitis/meningitis.

The new test works by detecting the levels of a particular type of antibody, IgM, to the disease in serum. IgM antibodies can be detected within the first few days of the onset of illness and can assist in the diagnosis of these patients.

The assay was evaluated using over 1000 patient sera tested at four different clinical sites. The test correctly identified antibody in 90-99% of cases. Because detection of antibody is not always specific in patients with acute viral infections, this test is considered presumptive and should be confirmed by more specific testing. Although the test is a valuable aid in the diagnosis of West Nile virus encephalitis, due to similarities with other viruses in the same family, there is a need to confirm positive results by an additional test or by using the current CDC diagnostic guidelines.

West Nile virus is a mosquito-borne virus which first appeared in the United States in 1999. While the virus often presents as a mild infection that clears without further treatment, some patients develop severe infection resulting in neurological disease and even death. The disease is most prevalent during the peak mosquito season. Over the past several years, the geographic range of the virus as well as the number of new infections has expanded and now covers most of continental United States.

Reference: FDA Talk Paper, P03-51. 9 July 2003.