How to Order a Test for West Nile Virus
Order serologic testing for WNV-specific IgM and IgG antibodies in serum, and if neuroinvasive disease is suspected, also test cerebrospinal fluid (CSF) for IgM antibodies. 1, 2
Primary Testing Approach
Standard Serologic Testing
- Order IgM and IgG antibody testing on serum as the first-line diagnostic test for suspected WNV infection 1, 2
- IgM antibodies become detectable 3-8 days after symptom onset and typically persist for 2-3 months, though they may remain positive for up to 12 months 1, 2
- Request both acute and convalescent sera (collected 7-10 days apart) to demonstrate seroconversion, which strongly suggests recent WNV infection 1, 2
Specimen Collection and Transport
- Collect serum in a clot tube or serum separator tube (SST) and transport at room temperature within 2 hours 1
- For CSF testing, collect in a sterile tube and transport at room temperature within 2 hours 1
When to Add CSF Testing
Order CSF IgM antibody testing when patients present with meningitis, encephalitis, acute flaccid paralysis, or other signs of neuroinvasive disease. 2, 3
- Detection of IgM antibodies in CSF indicates CNS infection, as IgM does not naturally cross the blood-brain barrier 1, 3
- Be aware that traumatic lumbar puncture or disrupted blood-brain barrier can cause false-positive CSF IgM results 1
Special Testing Situations
Immunocompromised Patients
Order nucleic acid amplification testing (NAAT/PCR) in addition to serology for immunocompromised patients, as they have delayed immune responses and prolonged viremia 1, 2
- Collect plasma (EDTA or plasma preparation tube) or serum (SST) and transport at room temperature within 2 hours 1
- NAAT is more sensitive than serology in patients with hematologic malignancies, solid organ transplants, or those receiving B-cell-depleting antibodies 4
Pregnant Women
- Do not screen asymptomatic pregnant women for WNV infection 2, 5
- Test pregnant women only if they present with meningitis, encephalitis, acute flaccid paralysis, or unexplained fever in areas with ongoing WNV transmission 2, 5
- Order both serum and CSF antibody testing in symptomatic pregnant women 2, 5
Confirmatory Testing
When Cross-Reactivity is Suspected
Request plaque reduction neutralization test (PRNT) for specimens that test positive for WNV antibodies when there is possible exposure to other flaviviruses (dengue, yellow fever, St. Louis encephalitis) or following yellow fever vaccination 1, 4
- False-positive IgM and IgG results occur due to cross-reactivity with other flaviviruses 1
- PRNT is also recommended for atypical presentations, deaths, or suspected unusual transmission modes like organ transplantation 4
- Only 25% of laboratories currently have capacity to perform PRNT, so coordinate with state public health laboratories if needed 6
Interpretation Pitfalls
Avoid These Common Errors
- Do not interpret isolated IgG positivity as acute infection—this indicates prior WNV infection and requires evaluation for alternative diagnoses 1, 2
- Do not rely solely on serology in severely immunocompromised patients—add NAAT/PCR testing 1, 2
- Do not order viral culture—it is insensitive and not routinely available 1
Reporting Requirements
Report all confirmed WNV infections to your state or local health department for surveillance purposes 1, 2