West Nile Virus Antibody IgG & IgM (Sendout)

Interpretation:

The presence of IgG-class antibodies to West Nile virus (WNV) in serum indicates infection with WNV at some time in the past. By 3 weeks postinfection, virtually all infected persons should have developed IgG antibodies to WNV. If acute-phase infection is suspected, serum specimens drawn within approximately 7 days postinfection should be compared with a specimen drawn approximately 14 to 21 days postinfection to demonstrate rising IgG antibody levels between the 2 serum specimens.

Presence of specific IgM-class antibodies in a serum specimen is consistent with acute-phase infection with WNV. By the 8th day of illness, most infected persons will have detectable serum IgM antibody to WNV; in most cases it will be detectable for at least 1 to 2 months following disease resolution and in some cases will be detectable for 12 months or longer.

The absence of IgM antibodies to WNV is consistent with lack of acute-phase infection with this virus. Specimens collected too early in the acute phase (eg, before 8 to 10 days postinfection) may be negative for IgM-specific antibodies to WNV. If WNV is suspected, a second specimen collected approximately 14 days postinfection should be tested.

In the very early stages of WNV infection, IgM may be detectable in cerebrospinal fluid (CSF) before it becomes detectable in serum.

Cautions:

Test results should be used in conjunction with a clinical evaluation and other available diagnostic procedures.

The significance of negative test results in immunosuppressed patients is uncertain.

Positive test results may not be valid in persons who have received blood transfusions or other blood products within the past several months.

False-negative results due to competition by high levels of IgG, while theoretically possible, have not been observed.

False-positive results may occur in persons vaccinated for flaviviruses (eg, yellow fever, Japanese encephalitis, dengue)

False-positive results may occur in patients infected with other arboviruses, including flaviviruses (eg, dengue virus) and alphavirusis (eg, LaCrosse [California] Encephalitis virus, Eastern or Western equine encephalitis virus, St. Louis virus) and in persons previously infected with West Nile virus (WNV).

Because closely related arboviruses exhibit serologic cross-reactivity, it sometimes may be epidemiologically important to attempt to pinpoint the infecting virus by conducting cross-neutralization tests using an appropriate battery of closely related viruses.

Outside Laboratories: Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum.

Stability: Refrigerated (preferred): 14 days; Frozen: 14 days; Ambient: Unacceptable.

Reject Due To: Gross hemolysis, gross lipemia, gross icterus, heat-inactivated specimen.

Centrifuge sample and transfer serum to a separate plastic vial. Refrigerate serum.

Sendouts:

• Order Mayo Test: WNS.
• Interfaced: Yes.

Stability: Refrigerated (preferred): 14 days; Frozen: 14 days; Ambient: Unacceptable.

Reject Due To: Gross hemolysis, gross lipemia, gross icterus, heat-inactivated specimen.