West Nile Virus Testing

All specimens received for Arbovirus serology will be tested against antigens of the Flavivirus group1, which include West Nile Virus (WNV) and St. Louis Encephalitis (SLE).

Revised Surveillance and Testing Procedure (October 2007)

Two procedures are available:

• IgM antibody detection on single acute, convalescent serum or CSF utilizing ELISA (Enzyme-Linked ImmunoSorbent Assay); and/or
• IgM antibody detection on single acute, convalescent serum or CSF utilizing MIA (Microsphere Immunoassy).

Note: Testing for Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE), and LaCrosse/California Encephalitis Group is available only after consultation with the West Nile Virus Program (573-751-6113 or (toll-free) 1-866-628-9891).

Note: Due to low specificity, IgG antibody tests are not useful in the diagnosis of acute WNV infection. An IgG positive test result, either alone or in conjunction with an IgM negative test, is not diagnostic for acute infections and is NOT considered a case of WNV.  For this reason, IgG serology for WNV has been discontinued at the MO State Public Health Laboratory.

Specimen Collection

• The ideal timing to collect acute serum is 3 to 10 days after onset of symptoms.
• Collect CSF as soon as possible after onset of symptoms.
• Collect serum in a redtop Vacutainer tube.  Serum is the preferred specimen but whole blood will be accepted.
• Whole blood may be sent if no method is available for removing the serum.
• At least 1 ml of serum and 1.5 ml of CSF is required for serological testing.

Submission Form

• Click here to Complete the Test Request Form
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Shipping Container

• Shipping containers are available from the SPHL upon request. (Click here)
• Serum and CSF specimens should be stored at refrigerator temperature prior to shipment and shipped at room temperature.
• Make sure specimens are packed securely to prevent breakage.

Testing Results

Testing results will normally be available 3 to 10 days after specimen receipt.  Reporting times may be extended during high or low volume testing periods.

Serum and CSF:

• All serum specimens will be tested against an IgM antibody panel that includes WNV and SLE by EIA and/or MIA. If indicated, EEE, WEE, and LaCrosse/California will be performed by Fluorescent Antibody testing.
• CSF specimens will be tested for WNV and SLE IgM antibodies only.
• Upon completion of testing the results will be mailed to the submitter.

Results for IgM Serology by ELISA

Positive (CSF) - Indicates recent infection.
Positive (Serum) - Indicates infection.*

If there is a significant difference between the P/N value (>2X) for WN or SLE, this would be indicative of an infection for the virus with the higher reading.

Examples:

1. WNV P/N 24.0, SLE P/N 6.0 = Indicative of an infection with WNV.
2. WNV P/N 10.0, SLE P/N 36.0 = Indicative of an infection with SLE.     
3. WNV P/N 6.0, SLE P/N 4.0 = Indicative of infection with a Flavivirus, Convalescent sample needed to distinguish between WN and SLE if possible. 1

* West Nile Virus IgM antibody may remain for more than one year.  Thus it will be difficult to determine if a positive IgM (from serum) is from an acute WNV infection. In this case clinical correlation is required to determine infection.

• Equivocal - Results of borderline significance (Convalescent specimen required for proper interpretation).
• Negative - Results fail to indicate an infection with WNV or SLE.