Diagnosis is usually provided by testing IgM antibodies in plasma or cerebral spinal fluid (CSF). These antibodies are detectable 3-8 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.