Diagnosis is usually provided by testing IgM antibodies in serum and/or cerebral spinal fluid (CSF). These antibodies are detectable 3 to 8 days after symptoms begin. They can persist for 90 days or up to a year in rare cases. CSF IgM positives should be confirmed with plaque reduction neutralization tests (PRNTs) performed in state public health laboratories. Antibodies to other flaviviruses may cross-react with WNV. Other laboratory tests are usually normal when patients have WNV, although CSF may show a pleocytosis with lymphocytes.
A positive IgG test result must have acute and convalescent titers demonstrating a four-fold rise to be diagnostic. WNV can be cultured and RT-PCR can be used for diagnosis.
Blood for donation in the United States is tested for WNV using nucleic acid tests (NAATs) and positive units are discarded.
