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Test Code BLOD0772 West Nile Virus, IgM

Useful For

The diagnosis of an active infection with West Nile virus,  a mosquito-borne flavivirus.


Enzyme Immunoassay (EIA)

Additional Information

  • Serology is the recommended method of testing for WNV in both serum and cerebral spinal fluid (CSF), because viremia (as detected by PCR) is very transient.
  • Negative results on specimens drawn less than 9 days from date of onset should have a convalescent serum tested if active disease is suspected.

Performing Laboratory

Montana State Public Health Laboratory

Specimen Requirements

Specimen Type: Serum and/or CSF
Container/Tube: Red top or gold top (serum) , sterile screw-top container (CSF)
Specimen Volume: 1 mL each

Collection Instructions:  Send 1 mL serum and/or CSF refrigerated.  Do not send by pneumatic tube. Paired acute and convalescent specimens recommended.  Date of onset is required, and the city or county of patient’s residence is requested.

Specimen Transport Temperature


Specimen Stability

Specimen Type Temperature Time
Serum Ambient 8 hours
  Refrigerated 1 week


Reference Values

Negative (reported as positive or negative)

Significant results are telephoned to the submitter.

Certain specimens may be referred to the Centers for Disease Control in Fort Collins, Colorado for confirmation using more specific Plaque Reduction Neutralization tests, and equivocal (borderline) results may be reflexed to St. Louis Encephalitis IgM Serology.

Day(s) Test Set Up

Routinely batch tested once per week; during seasonal outbreaks, testing may be performed each working day, depending on workload.

Test Classification and CPT Coding