What is the recommended method for testing for West Nile virus (WNV) infection?

Diagnostic Testing for West Nile Virus

Serologic testing for virus-specific IgM and IgG antibodies in serum and/or cerebrospinal fluid (CSF) is the primary diagnostic method for West Nile virus infection. Detection of anti-WNV IgM antibodies in serum and CSF is the recommended first-line testing approach for suspected West Nile virus infection. 1, 2

Primary Diagnostic Approach

• Serologic testing should be performed on both serum and CSF samples when neuroinvasive disease is suspected 1
• IgM antibodies to WNV become detectable 3-8 days after symptom onset and typically decrease after 2-3 months, though they may persist in serum for up to 12 months 1, 2
• Seroconversion to anti-WNV IgM and/or IgG positivity between acute and convalescent sera (collected 7-10 days apart) strongly suggests recent WNV infection 1, 3
• The presence of anti-WNV IgM in CSF indicates CNS infection, as these antibodies do not naturally cross the blood-brain barrier 1, 3

Specimen Collection and Transport

• For serology testing: Collect serum in clot or serum separator tube (SST), transport at room temperature within 2 hours 1
• For NAAT testing: Collect CSF in sterile tube, plasma in EDTA or PPT, or serum in SST, transport at room temperature within 2 hours 1
• If initial testing is negative but clinical suspicion remains high, repeat testing is recommended, as the cumulative percentage of seropositive patients increases by approximately 10% per day during the first week of illness 1, 2

Special Considerations

• Nucleic Acid Amplification Testing (NAAT) has higher sensitivity in immunosuppressed patients due to delayed immune response and prolonged viremia 1, 2
• False-positive results for both anti-WNV IgM and IgG antibodies may occur in patients vaccinated against yellow fever virus or following natural infection with other flaviviruses (e.g., dengue, St. Louis encephalitis viruses) 1, 4
• To rule out cross-reactivity, specimens reactive for WNV antibodies should be tested by plaque reduction neutralization test (PRNT) 1, 5
• Viral culture, while possible, is insensitive and not routinely offered at local or reference laboratories 1

Diagnostic Algorithm

1. Collect both serum and CSF samples if neuroinvasive disease is suspected 1
2. Test for WNV-specific IgM antibodies in serum and CSF using ELISA or immunofluorescence assay 1, 6
3. If positive, confirm with PRNT to rule out cross-reactivity with other flaviviruses 1, 5
4. If initial testing is negative but clinical suspicion remains high, collect paired sera 7-10 days apart to detect seroconversion 1
5. Consider NAAT testing in immunosuppressed patients, as they may have delayed antibody response 1, 2

Common Pitfalls

• Relying solely on NAAT for diagnosis may lead to false-negative results, as viremia often precedes symptom onset 1, 2
• Failing to collect paired sera may miss seroconversion, which is strongly suggestive of recent infection 1, 7
• Not considering cross-reactivity with other flaviviruses, which can lead to false-positive results 1, 4
• Interpreting the presence of anti-WNV IgG alone as evidence of acute infection, when it actually indicates prior WNV infection 1, 3
• Traumatic lumbar puncture or defective permeability of the blood-brain barrier may lead to falsely elevated IgM levels in the CSF 1