This test is indicated for use as an aid to the diagnosis of West Nile virus encephalitis. WNV IgM antibodies typically appear three to eight days after illness onset and persist 30 to 90 days (but can persist up to 500 days). WNV IgG antibodies appear shortly after IgM antibodies and persist greater than 500 days. Due to cross-reactivity with other flaviviruses, confirmation by neutralizing antibody assay may be recommended.