The best test for WNV infection is detection of IgM antibodies to the virus.

1. The University Hygienic Laboratory (UHL) uses the WNV IgM-antibody test developed by CDC. This test has been shown to be highly specific and there are very few false positive results.
2. IgM antibody is typically produced shortly after symptoms occur. The best specimen is serum and/or cerebral spinal fluid (CSF), in the case of neuroinvasive disease, collected three to 14 days from symptom onset.
3. WNV IgM antibodies typically peak at eight days post symptom onset.
4. Submit with UHL test request form. Patient history is a valuable aid in patient history. Complete specimen collection instructions are avaliable here.

Interpretation of Test Results:

1. All test results must be interpreted in conjunction with clinical and epidemiologic information.
2. Demonstration of WNV IgM antibody in CSF is considered diagnostic confirmation of WNV infection and strongly suggests neuroinvasive disease.
3. Demonstration of WNV IgM antibody in serum is diagnostic for a probable WNV infection; however, false positives and cross reactivity can occur especially in patients recently vaccinated against Yellow Fever or infected with related Flaviviruses (e.g., St. Louis, Yellow fever, Japanese encephalitis, Dengue).
4. WNV-specific IgM has persisted in patients for >500 days, positive serologic tests must be considered in relation to clinical presentation. An equivocal test result suggests a questionable presence of West Nile Virus IgM antibody detected. Repeat testing in 10-14 days may be helpful for specimens collected very early in the disease process.
5. An indeterminate result may occur due to a cross reaction by heterophile antibodies in the patient's serum. In this case the result is reported as indeterminate. Heterophile antibodies are non-specific antibodies that react with antigens in the test kit.